Gemara

122b (R. Avahu citing Reish Lakish): Four Mil is the Shi'ur for Tefilah.

(R. Yosi bar Chanina): Four Mil is the Shi'ur in the direction one was going, but one need not backtrack even one Mil.

Inference (Rav Acha bar Yakov): If he can backtrack less than one Mil, he must do so.

Berachos 6b (Ravin bar Rav Ada): If one normally comes to the Beis ha'Keneses, and one day he does not come... If he went for Reshus (not for a Mitzvah) he will not have Nogah (light). He should have trusted in Hash-m.

14a (Rav Idi bar Avin): One may not do his needs before prayer -- "Tzedek Lefanav Yehalech v'Yasem l'Derech Pe'amav."

(Rav Idi bar Avin): If one embarks on his journey after praying, Hash-m will do his desires -- "Tzedek Lefanav Yehalech..."

15a (Ravina): A Chacham from Eretz Yisrael said that one who does not have water should clean his hands in earth, pebbles or chips of wood!

Support (Rava): It does not say "I will wash in water," rather, "b'Nikayon", i.e. in anything that cleanses!

Rav Chisda would curse one who delays praying until he can wash his hands with water.

This refers only to Keri'as Shma. For Tefilah, he should go up to four Mil in the direction that he was traveling; or one Mil in back.

Rishonim

Rif and Rosh (Berachos 8a and 2:7): One may not embark on a journey before prayer. If one embarks on his journey after praying, Hash-m will do his desires.

Ma'adanei Yom Tov (100): Our and Rashi's text say that the Isur is to do his needs before prayer.

Rif (ibid.) Rav Chisda would curse one who delayed Kri'as Shma until he found water. For Tefilah, he must go up to four Mil in front, or one Mil in back.

R. Yonah (DH Layit): This is in some texts of the Gemara. It is not in old accurate texts. Chachamim of France say that it was inserted by Ge'onim, but it is worng. 'Prayer' refers to Shemoneh Esre. Rashi explained that one must go four Mil or one Mil to pray with a Minyan. Those who have the text hold that we are more stringent about Tefilah than Kri'as Shma, for Tefilah is only mid'Rabanan.

Rosh (2:11): Some texts are like the Rif's text. This is unlike old texts and Rashi's text. Just like one does not pursue water for Kri'as Shma, also for Tefilah. Also, the Gemara (22a) says that they nullified washing, like Rav Chisda taught. It did not mention (that this is only for) Kri'as Shma.

Rambam (Hilchos Tefilah 4:2): Before Tefilah, one must wash his hands until the wrist. If one is on the road, he must pursue water if it is available in front of him within four Mil. If he is further than four Mil from water, he cleans with a pebble, earth or board and prays.

Rashi (15a DH li'Kri'as): We do not pursue water at the time of Kri'as Shma, for it has a fixed time and one might miss the time. We pursue water for Tefilah, for the entire day is Zman (the time for) Tefilah.

Poskim

Shulchan Aruch (OC 89:3): One may not engage in his needs or go on the road before praying Shemoneh Esre.

Beis Yosef (DH u'Mah she'Chasav Oh): This is from the Tur. Mahari Avuhav disagreed, for later the Tur brings two opinions. Some say that it is better to pray (on the road) in the proper time than to pray in the house. Here the Tur holds like the Rosh (4:20), that it is better to pray in the house. I disagree. There, we discuss when the caravan is leaving, and one cannot wait until Zman Tefilah. Here, he could wait, but he wants to start traveling promptly before praying. The Tur forbids this. He holds that this is included in 'pursuing your needs before Tefilah.' If one embarks on a journey after praying, Hash-m will do his desires (Berachos 14a). Do not infer that if he did not pray first, Hash-m will not do his desires, but there is no Isur. Rather, it is forbidden to travel first, just like pursuing one's needs before Tefilah.

Taz (3): Here we discuss going even a short distance. One will reach his destination with enough time to pray standing. Even so, it is forbidden due to pursuing one's needs before Tefilah. Below, the Tur discusses one who leaves before Zman Tefilah, but he would need to pray on the road, not standing.

Magen Avraham (10): I say that if the caravan will not wait for him, he may go (before Tefilah).

Mishnah Berurah (19): If the time for Minchah or Ma'ariv came, one may not travel before praying, just like for Shacharis.

Mishnah Berurah (20): Even if he will be able to pray with a Minyan in the proper time in his destination, it is forbidden, for traveling is like pursuing one's needs. However, in pressed circumstances, if the caravan will not wait for him, he may go and pray on the road. Many Poskim permit even after sunrise.

Kaf ha'Chayim (27): One may begin traveling before Zman Tefilah. The Zohar mentions that R. Shimon and R. Yehudah were on the road, and the time for Kri'as Shma came. One must begin before dawn.

Rema: Some are lenient after saying some Berachos before Baruch she'Amar. It is good to be stringent.

Gra (DH v'Tov): The Isur to greet with 'Shalom' is until one prays Shemoneh Esre. The same applies to the Isur to travel before Tefilah, for they were taught together (Terumas ha'Deshen 18).

Shulchan Aruch (90:16): If one is on the road and reached a city and wants to lodge there, if there is in front of him within four Mil a place where they pray with 10, he must go there.

Magen Avraham (30): This is if he can reach there in when much time remains in the day, and he will not need to go alone at night.

Mishnah Berurah (51): 'Much time remains in the day' means before sunset. If not, he is exempt, for one should enter a city during the day.

Mishnah Berurah (49): The same applies in the morning. If in the direction he is going there is a Minyan within four Mil, he waits until reaching there, provided that he will not miss the time for Kri'as Shma and Tefilah.

Mishnah Berurah (50): This is only in front, since in any case he must go there. Going to the side is like going back.

Shulchan Aruch (ibid.): In back, he must go until one Mil in order to pray with a Minyan.

Mishnah Berurah (52): If one is in his house, this is like going back. Therefore, if one lives in a settlement within one Mil of a Minyan, he must go there every morning to pray with a Minyan. He need not go at night, for one need not go alone at night to pray with a Minyan. This Sa'if rebukes people in a city who are too lazy to go to the Beis ha'Keneses to pray Minchah and Ma'ariv.

Kaf ha'Chayim (107): We do not measure a Mil from his house, for if so one who lives in a big city would be exempt if the Minyan is on the other side of the city! Rather, if the Minyan is within the Techum, i.e. one Mil outside the city, he must go.

Shulchan Aruch (17): Some say that all the more so one may not rise early to leave a city in which there is a Minyan, if one (could stay to pray) and reach his destination with much time left in the day and he will not need to go alone after Tefilah.

Beis Yosef (DH Matzasi): The Agudah (Chulin 9:149) learns this from a Kal va'Chomer from the law that if there is a Minyan within four Mil one waits (to pray there).

Kaf ha'Chayim (108): We do not find that anyone disagrees. The Mechaber writes 'some say' when only one Posek taught the law.

Magen Avraham (31): The Gemara (6b) says that if one went for Reshus, Hash-m will not wait for him. That refers to one who went to do his affairs in the city. Chachamim were not so stringent about one who goes on the road.

Eshel Avraham: It seems that here it is forbidden only if traveling will cause him to lose Tefilah b'Tzibur. Above (89:3), we always forbid traveling before praying Shemoneh Esre!

Mishnah Berurah (53): We discuss one who went for Reshus. The Eshel Avraham's question is not difficult for the Rema. After one said Berachos, he may leave unless he will miss Tefilah b'Tzibur.

Shulchan Aruch (92:4): Before Tefilah, one must wash his hands with water, if he has.

Mishnah Berurah (13): One must wash also for Minchah and Ma'ariv. The Rambam holds that if one needed to wash and he did not wash or clean his hands, he was not Yotzei even b'Di'eved. This requires investigation. I concluded that one should not pray again.

Mishnah Berurah (14): Drying the hands is not Me'akev, unlike washing for a meal, in which it is Me'akev.

Shulchan Aruch (ibid.): If one is on the road, he must pursue water if it is available in front of him within four Mil, or one Mil in back.

Beis Yosef (DH v'Rabeinu): The Tur (90) explained like Rashi's text. Why did he bring this law? He holds that Rav Chisda does not curse one who seeks water after Zman Tefilah came. It is a Mitzvah. One should pursue it! He cursed one who pursues it when this could cause missing Zman Tefilah. Mordechai says in the name of Tosfos that one should not miss Zman Tefilah for the sake of water. If he will not miss Zman Tefilah, he should pursue water, just like other Mitzvos. Since the Rif, Rambam and Rashba in the name of Rav Hai Gaon distinguish Kri'as Shma from Tefilah, we hold like them.

Kaf ha'Chayim (18): The Mechaber rules like the Rif. Why does he rule like Tosfos in a case when he will miss Zman Tefilah? The Drishah left this difficult. Divrei Chamudos (2:38) answered that it is like a compromise. He did not want to rule like the Rif in a case that one will miss Zman Tefilah.

Gra (DH Im): If one would miss Zman Tefilah, he relies on Tosfos' text (that Tefilah is like Kri'as Shma). According to the Ge'onim, one must go four Mil even if he will miss Zman Tefilah. Rashi connotes like this.

Kaf ha'Chayim (19): The Bach says that since we do not wait for water for Kri'as Shma, and Shemoneh Esre must be Samuch (right after) Kri'as Shma, one does not pursue water for Shemoneh Esre of Shacharis. This is unlike the Mechaber. Many explain that one can say Kri'as Shma and delay until he gets water.

Mishnah Berurah (15): One may be lenient if he does not know that his hands are dirty, like in Sa'if 5.

Mishnah Berurah (16): One need not pursue water if afterwards he will need to go alone or not reach his destination during the day.

Shulchan Aruch (ibid.): If he fears lest Zman Tefilah pass, he cleans with a pebble, earth or board, and prays.

Magen Avraham (3): One must wash whether his hands are dirty, or if he had Hesech ha'Da'as (ceased thinking about guarding them). How does a pebble help for Hesech ha'Da'as? Perhaps sweat or excrement clung to them, and the pebble will clean it.

Mishnah Berurah (20): One does not pursue water if this could cause missing Tefilah b'Tzibur.