Gemara

50a (Rav Huna): If three came from three groups, they must make a Zimun together.

51b (Mishnah - Beis Shamai): If someone ate and forgot to bless, he must return to where he ate and bless there;

Beis Hillel permit him to bless in his current place.

53b (Rav Zevid): They argue about one who forgot to bless. If he intentionally left without blessing, all agree that he must return to his place.

Rabah bar bar Chanah was traveling with a caravan. He forgot to bless. He reasoned that if he tells them, they will say "[we need not wait for you to return.] You can bless Hash-m anywhere!"

He told them that he forgot a golden dove. They agreed to wait for him. He returned, blessed, and found a golden dove.

Rishonim

Rambam (Hilchos Berachos 4:1): One must bless Birkas ha'Mazon or me'Ein Shalosh in the place where he ate. If one forgot to bless, and remembered before the food was digested, he blesses in his current place.

Hagahos Maimoniyos (1): Also the Rashbam, R. Simchah and Rashi say that all seven species requires a Berachah in their place. However, the Ri and Rif say so only about bread, and I heard so in the name of Rav Amram Gaon.

Rosh (8:5): This is only for Birkas ha'Mazon. One need not bless on the seven species in their place. He blesses where he is when he remembers.

Ra'avan (188): If one forgot and did not bless Birkas ha'Mazon, if he is in the city he returns to where he ate. If he is on the road and did not remember until he started traveling, he blesses where he remembered.

Eliyahu Rabah (2): What is the source to distinguish like this? The episode with Rabah bar bar Chanah was on the road! Also, if so Rav Zevid should have distinguished between one who ate on the road or in the city! Also, Bahag, Semag and Semak say 'one who forgot and did not bless and went on the road or to the market...' This connotes that we do not distinguish. It seems to me that Ra'avan was unsure whom the Halachah follows, therefore he himself decided to rule like Beis Shamai in the city, where it is not an exertion, and like Beis Hillel on the road, for it is an exertion and one's mind is not settled when he is delayed. It seems that the same applies if he ate in the city and went on the road. He blesses where he remembered. The Ra'avan mentioned eating in the city and on the road, for this is a typical case.

Nishmas Adam (2, on Chayei Adam 47:9): We must distinguish like Ra'avan says. The episode with Rabah bar bar Chanah proves this. According to those who rule like Beis Shamai, why was he concerned lest they tell him to bless here? He could respond that the Halachah follows Beis Shamai! And why would he assume that they will tell him to bless here? Rather, we must say that those who rule like Beis Shamai say so only in the city. On the road, all agree, i.e. even Beis Shamai, that he need not return. Rabah wanted to be stringent.

Note: Perhaps Rabah assumed that they will assume that the Halachah follows Beis Hillel, and he could not prove that it follows Beis Shamai.

Poskim

Shulchan Aruch (OC 184:1): One who ate in one place must bless before he leaves his place. If he left his place and did not bless, if he was Mezid, he returns to his place and blesses.

Magen Avraham (1): Moving from one corner to another is not called changing place (178:1). The same applies if one can see his initial place. If one eats here with intent to bless afterwards elsewhere, perhaps this helps. One may rely on this when his initial place is not clean.

Mishnah Berurah (1): One may move from one corner to another even in a big house, even if he cannot see his initial place. If he cannot bless in the room he ate in, he may bless in an adjacent room if he can see where he ate. If when he blessed ha'Motzi he intended to bless in another room in the house, perhaps one may be lenient in pressed circumstances, even if he cannot see his initial place.

Kaf ha'Chayim (1): One should not bless in a different corner without need.

Kaf ha'Chayim (3): The Heter to bless in a different room when he ate in a dirty room is only if he could not eat in the clean room (Machatzis ha'Shekel).

Mishnah Berurah (2): Within four Amos is considered one place.

Magen Avraham (2): If he is far away, and before he return to his place the food will have been digested, he blesses here (where he remembered).

Chayei Adam (47:9): If one left b'Shogeg, in pressed circumstances he need not return, and he blesses in his new place. It seems that on the road, even if he ate in an inn, if he left b'Shogeg, he need not be stringent at all.

Mishnah Berurah (3): Even if he already traveled several kilometers, he must return, unless the food would be digested first and he would lose the Berachah Acharonah.

Shulchan Aruch (ibid.): If he was Shogeg, the Rambam holds that he blesses in his current place, and R. Yonah and the Rosh say that also he returns to his place and blesses.

Beis Yosef (DH v'Chasav): The Rosh says that one returns to his place only for Birkas ha'Mazon. One need not bless on the seven species in their place. The Rosh did not mean only Birkas ha'Mazon, rather, species on which one blesses Birkas ha'Mazon, i.e. the five grains. Since they are Zan (sustain), he calls the Berachah on them Birkas ha'Mazon, even though he blesses only me'Ein Shalosh. In Pesachim (10:6), the Rosh wrote that bread and the grains require a Berachah in its place. Here, he said that one need not bless on the seven species in their place. He refers to the five species other than wheat and barley. However, the Rashba (53b DH Machlokes) says that anything that requires only one Berachah (afterwards), one need not bless on it in its place. Some say that all seven species require a Berachah in their place. For all of them, one must return to his place and bless.

Mishnah Berurah (7): One should follow the latter opinion, unless it is pressed circumstances. Then, one relies on the first opinion.

Shulchan Aruch (2): This is when he does not have more bread. If he has more bread, he eats a little in the new place and blesses, as long as he is not hungry from the first eating.

Beis Yosef (DH Kasav ha'Rav): R. Peretz says so in his Hagahos to Semak. Hagahos Maimoniyos brings this.

Magen Avraham (1): In Siman 178, the Beis Yosef said that this is like those who rule like R. Chananel. The Rambam and his followers hold that since he may not eat more without blessing first, surely he must return to his initial place to bless on what he ate. There, the Beis Yosef ruled like the Rambam. If so, why does he say here (that he eats more)? I answer that since he left b'Shogeg, there was no Hesech ha'Da'as (he did not divert his mind from eating), so he may eat without Birkas ha'Mazon. It seems to me that if he has bread, he eats here and blesses beforehand, and after eating he blesses a second time, and he is Yotzei with according to all opinions. According to the Rosh he is Yotzei with the second Birkas ha'Mazon, and according to the Rambam he is Yotzei with the first. The Toras Chatas (30:13) wrote similarly to this. According to the Rosh, the first Birkas ha'Mazon is l'Vatalah! (For that law,) the Beis Yosef rules like the Rambam, but in this Halachah he did not decide the matter.

Gra: We learn from three who came from three groups (50a). We must say that they were in different places, for if not they join even without (eating more now). Rashi says that they join even if they did not eat together. I.e. they did not eat together at the same time, but each of them ate by himself (in their new place).

Damesek Eliezer: This proves that they eat more now and bless here, and they need not return to their initial places.

Mishnah Berurah (8): Even if he left b'Mezid, it helps to eat a little bread here. He need not bless ha'Motzi, nor Birkas ha'Mazon on what he ate beforehand. After eating here, he says Birkas ha'Mazon for what he ate there and here.

Kaf ha'Chayim (8): L'Chatchilah one may not leave his place with intent to eat more in the new place and bless there.

Mishnah Berurah (9): Even if he eats less than a k'Zayis now, it joins with what he ate before. He must eat bread now. The Pri Megadim leans to say that even a k'Zayis of accompaniments or Mezonos does not help.

Kaf ha'Chayim (10): Divrei Chamudos (8:34), Shirei Keneses ha'Gedolah, the Magen Avraham and Chayei Adam say that he need not eat a k'Zayis. Hilchos Ketanos says that he must eat a k'Zayis, for less than this is not important.

Mishnah Berurah (10): If he is hungry now, he already lost Birkas ha'Mazon on the first eating, and now is a new obligation. He must bless also ha'Motzi.

Kaf ha'Chayim (9): If he had decided not to eat more, he must bless ha'Motzi before eating more.

Shulchan Aruch (3): Some say that all seven species require a Berachah Acharonah in their place. Some say that only the five grains do.

Magen Avraham (6): If he left his place, the Halachah is like in Se'ifim 1 and 2.

Rema: See \178:5.

Mishnah Berurah (12): There, the Rema brought a third opinion, that one must go back only for bread. The Gra wrote that the primary opinion is that only the five grains are like bread.

Kaf ha'Chayim (14): The Mechaber rules like one must return only for the five grains. However, l'Chatchilah one should be stringent for all seven species.