Gemara

(Beraisa): One may choose to hold like Beis Shamai, or like Beis Hillel. If one holds like the leniencies of both, he is wicked. If one holds like the stringencies of both, he is a fool walking in darkness;

Rather, one must hold consistently like Beis Shamai, or like Beis Hillel.

Menachos 32a (Beraisa): If parchments of Tefilin became worn, they may not be used for a Mezuzah, for we do not demote something to a lower Kedushah.

Inference: If it were permitted to demote, they could be used for a Mezuzah.

32b - Question: Mezuzah requires Sirtut (lines scratched in the parchment to help the scribe write straight. Tefilin do not require Sirtut!

Answer: Tana'im argue about whether a Mezuzah requires Sirtut.

Sukah 26b (Mishnah): A case occurred in which they brought a cooked food for R. Yochanan ben Zakai to taste, and they brought two dates and water for R. Gamliel, and they said 'bring them up to the Sukah.' When they gave to R. Tzadok (bread) less than k'Beitzah, he wrapped his hands in a cloth and ate it outside the Sukah.

Question: The case contradicts the law!

Answer: The Mishnah is abbreviated. It means as follows: if one wants to be stringent, he may be stringent, like R. Yochanan ben Zakai and R. Gamliel.

Yoma 79a (Beraisa): Yochanan ben Zakai and R. Gamliel were exempt. They were stringent on themselves.

79b (Rava): They were exempt because one need not eat Peros in a Sukah.

Rishonim

Rambam (Hilchos Sukah 6:3): A Chasan, his friends (who help make the wedding) and all the Bnei Chupah are exempt from Sukah all seven days of feasting.

Hagahos Maimoniyos: R. Simchah says that anyone who is exempt and does not leave the Sukah is called a Hedyot.

Rambam (6): One may drink water or eat Peros outside a Sukah. If one is stringent and does not drink even water outside a Sukah, this is praiseworthy.

Ran (Sukah 11a DH Ma'aseh): The case of R. Tzadok, who was not stringent, teaches that that if a Chacham or Talmid is not stringent, he is not considered one who is not meticulous about Mitzvos.

Rosh (Hilchos Sefer Torah 7, citing R. Tam): Why did the Gemara need to say that Tana'im argue about whether a Mezuzah requires Sirtut? We could have said that all exempt, and we discuss one who did Sirtut even though he is exempt! He answered that the Gemara did not want to say so, for anyone exempt from a matter, yet he does it, is called a Hedyot.

Poskim

Shulchan Aruch (OC 32:6): One need not be Mesartet more than the first line (of Tefilin). If one does not know how to write straight without Sirtut, he should be Mesartet all the lines.

Magen Avraham (6): In some texts there is a comment of the Rema 'one need not (be Mesartet)', i.e. for (the Shulchan Aruch said one (should) not be Mesartet, and the Rema emended this to say 'need not.' The Bach disagreed, and said that one should not.

Magen Avraham (8): The Yerushalmi says that if one is exempt and fulfills, he is called a Hedyot, i.e. if he does something for a stringency. Here he does not intend for this. This is not an Isur to which a stringency applies. He does Sirtut only in order to write straight. He is not a Hedyot.

R. Akiva Eiger: This has no connection to the Mechaber's law! The Mechaber discusses one who cannot write straight without Sirtut. He is obligated to do Sirtut. He is not exempt! The Beis Yosef (below) comments on the Tur, who allows Sirtut even for one who does not need it.

Beis Yosef (OC 32 DH u'Mah she'Chasav Rabeinu v'Im): Why does the Tur permit to do Sirtut? The Seforim say that he is called a Hedyot! Perhaps he learned from Sefer ha'Terumah, who holds like R. Tam, but then cited his Rebbi, who said that one is not called a Hedyot if he does Sirtut also regarding Tefilin, in order to write straight. It is not a stringency lest he transgress. He holds that the Yerushalmi says that if one is exempt and fulfills, he is called a Hedyot only when he does so to be stringent. Even if he can write straight without Sirtut, if he writes nicer with Sirtut, he is not called a Hedyot.

Be'er Sheva (Teshuvah 21): Perhaps the Yerushalmi says that he is called a Hedyot only when he does so to be stringent. Sefer ha'Terumah says so about Sirtut in Tefilin. He is not called a Hedyot if he does so in order to write straight, but not due to a stringency. The same applies to similar cases!

Magen Avraham (8): This requires investigation. We accept many stringencies on ourselves, and we do not say that we are called Hedyotos! Also Be'er Sheva questioned the Beis Yosef.

Eshel Avraham: Regarding Sukah, even though the Halachah is lenient, since there is a stringent opinion, he is not called a Hedyot.

Rema: Some say that always one must be Mesartet above and below and to the sides, even if he knows how to write straight without Sirtut. This is the custom.

Bi'ur Halachah (DH v'Chen): Nowadays, the custom is to be Mesartet all the lines.

Shulchan Aruch (639:2): One may eat Arai (not in a fixed manner) outside a Sukah, i.e. a k'Beitzah of bread. One may drink water or wine or eat Peros (Rema - even if he fixed his meal on them). One who is stringent and does not drink even water outside a Sukah, this is praiseworthy.

Beis Yosef (DH u'Mah she'Chasav Aval): The Rif did not bring that Peros do not require a Sukah. The Ran (Sukah 11b DH Ma'aseh) says that he holds that the Halachah does not follow this answer. The Rambam learns from R. Yochanan ben Zakai and R. Gamliel, who went to the Sukah to taste a cooked food or consume two dates and water. The Gemara explains that one may be stringent. I.e. he is not considered like one who is exempt and fulfills, who is called a Hedyot.

Taz (6): The cases with R. Yochanan ben Zakai and R. Gamliel teach that if one is stringent, this is not haughtiness.

Sha'arei Teshuvah (5): R. Yerucham said that there is no reason to be stringent for less than k'Beitzah. Eliyahu Rabah brings that Shibolei ha'Leket says that on Shabbos and Yom Tov, even eating Arai is considered fixed. If so, what was the question about R. Gamliel? The Poskim do not distinguish Shabbos and Yom Tov from Chol ha'Mo'ed regarding Leshev ba'Sukah.

Mishnah Berurah (13): L'Chatchilah it is proper not to drink even other drinks such as mead and beer only amidst a meal (for there is a Safek whether to bless Leshev ba'Sukah). Or, at least one eats first k'Beitzah of bread (or Mezonos, if he fixes his meal on it) and blesses Leshev ba'Sukah, and fulfills all opinions.

Mishnah Berurah (15): Several Acharonim agree that it is proper to be stringent and not eat meat, fish or cheese outside the Sukah. This is only if one fixed to eat them. If not, one should not be stringent at all.

Kaf ha'Chayim (37): Birkei Yosef brings from Tzedah l'Derech that water is an excessive stringency. The Rambam explains that the stringency was also due to the water. Tzedah l'Derech explains that it was due only to the dates. However, since the Rambam and Shulchan Aruch say so, we follow them.

Kaf ha'Chayim (38): All the more so it is praiseworthy to be stringent about bread eaten Arai, wine and Peros.

Rema: Some say that nowadays people are lenient not to sleep in the Sukah, except for those meticulous in Mitzvos, because it is painful in cold places.

Mishnah Berurah (17): If one does not have enough pillows and blankets and it is cold, he should not be stringent to sleep in the Sukah. He is called a Hedyot.

Rema (7): If one was sleeping in a Sukah and any amount of rain came, he may leave. Anyone who is exempt from Sukah and does not leave does not receive reward. He is called a Hedyot.

Taz (19): All the more so, if it is raining and one goes to the Sukah to bless ha'Motzi and Leshev ba'Sukah and afterwards eat in the house, it is a Berachah l'Vatalah.

Bi'ur Halachah (DH v'Chol): He is called a Hedyot only when there is an aspect of Isur, e.g. one who is in pain, for this desecrates Yom Tov. One must honor even Chol ha'Mo'ed. If he is not in pain, he may be stringent, like R. Gamliel.

Kaf ha'Chayim (89): He is called a Hedyot because he does not fulfill the Mitzvah of Sukah, and he desecrates Yom Tov.

Bi'ur Halachah (DH Hedyotos): Bikurei Yakov says that he is called a Hedyot and he does not receive reward only when he is exempt at the time he sits in the Sukah, especially when it is raining. Since his Master showed that he does not want his servant's service, and he wants to forcibly serve his Master, this is improper. If one is exempt due to toil to return to the Sukah, e.g. the rain stopped at night or during his meal, or to go to another's Sukah, this is praiseworthy, even though letter of the law he is exempt. While in the Sukah he fulfills the Mitzvah properly, just l'Chatchilah he was not obligated to go to another's Sukah. This is why Hagahos Maimoniyos discussed one who is exempt and does not leave the Sukah. This implies that he is exempt at the time he sits there.

Rema (640:4): If the Neros in the Sukah extinguished on Shabbos, and he has light in the house, he may leave the Sukah to eat where there is light. He need not go to another's Sukah where there is light if this is a big toil.

Mishnah Berurah (22): It is considered painful, for a person likes to eat only in his own house. The Chayei Adam says that in such a case of pain, one may obligate himself and be happy, and bless (Leshev ba'Sukah).

Taz (OC 472:10): Whenever we say that one need not lean at the Seder, it is forbidden to lean. This is like we say about Sukah. If one is exempt and fulfills, he is called a Hedyot. The Maharshal says so.