עוסק במצוה פטור מן המצוה

MISHNA: Those on the path to perform a mitzva are exempt from the mitzva of sukka. The ill and their caretakers are exempt from the mitzva of sukka. One may eat and drink in the framework of a casual meal outside the sukka.

GEMARA: The Gemara asks: From where are these matters derived that one who is performing a mitzva is exempt from the mitzva of sukka? The Sages taught in a baraita that it is written in the Torah that one recites Shema at the following times: “When you sit in your house, and when you walk by the way, and when you lie down, and when you rise up” (Deuteronomy 6:7). The Sages interpret: “When you sit in your house,” to the exclusion of one who is engaged in the performance of a mitzva, who is not sitting at home; “and when you walk by the way,” to the exclusion of a groom, who is preoccupied with his mitzva of consummating the marriage and is not walking along the way. The baraita adds that from here the Sages stated: One who marries a virgin is exempt from reciting Shema on his wedding night, and one who marries a widow is obligated.

Gemara: In your sitting in your house exempt one involved in a mitzvah - ...that he is not obligated in reading the Shema and later it is explained what the implication is.

And in your going in the road exempts a groom - ...and even though he is also occupied in a mitzvah, a second verse is needed for him. For if the first had been written, since a groom was not explicitly written in the verse, I would have thought that the verse only exempts one who is burdening and doing the work of a mitzvah with his hands, or traveling for a mitzvah, but a groom, who sits idle and is not involved except for his preoccupation in contemplating marital relations, is not exempted by the verse. Therefore, the additional text teaches us that he is exempt from reading the Shema, for he is preoccupied in contemplating about marital relations.

The messengers of Mitzvah ... Those who travel to fulfill a Mitzvah, for example to learn Torah or to greet his Torah teacher and to redeem captives are exempt from Suka, and even at the time of their encampment (to rest). So is explained in the notes of Rash"i and so is implied by the Gemora because it says: "Those who travel to fulfill a Mitzvah are exempt from Suka, whether during the day or during the night". It implies (that they are exempt from Suka) even if they don't travel except during the day. Because, if (the Gemora was talking about the case where) they went on traveling during the day and during the night, even (if they would have gone) for a permitted non-Mitzvah activity, they would be also exempt (from Suka), as was thought in a Brisa. And also (another example of being exempt from Suka even when not directly involved in doing a Mitzva), a story with Rav Chizda and Raba bar bar Huna that were resting on the shore of Sura river (outside of Suka), (when they came to hear a Torah exposition and to visit the Leader of the Exile - Rash'i). Toisfos continue: and it is a wonder, if they are able to fulfill both (i.e. the Mitzvah that they are currently involved in, and the Mitzvah of Suka), why are they exempt (from Suka)? Just because a person has Tzitzis on his garment and Tfilin on his head, who would exempt him with this from the rest of the Mitzvahs? (obviously nobody). Toisfos continue: and in the chapter "Ein Bein Hamoder" [Nidorim chapter 4, page 33b], in regard to the one who swore against getting benefit (from the owner of the lost object), that he (the finder, nevertheless,) returns to him (to the owner of the lost object) his lost object, even though he (the finder) benefits a Pruta (a small amount of money, according to the opinion) of Rav Yosef. (Rav Yosef holds in Meseches Bava Kama, bottom of the page 56b that a finder of a lost objects benefits from the lost object by being exempt from giving a piece of bread to a poor man who knacks on his door while the finder is involved in the Mitzvah of lost object, thus, saving to the finder the value of that piece of bread). (The finder, despite of getting the benefit, should return the object) because it is not a frequent event (that a poor man would knock on the door while the finder is involved in the Mitzvah of lost object). (However) if all the time that the lost object is in his (finder's) house, he (the finder) is exempt from giving bread to a poor man, if it is so, it is a frequent event (saving the value of a piece of bread, because poor people knocking on the door, while the object is in his house, are frequent). Rather, for sure, his is not exempt (from giving bread) except at the time that he is involved with it (with the lost object). For example, lost Talis and he (the finder) spreads it for its needs (to air it out), or an animal when he gives to it food. It is not frequent that at that moment would come a poor man to ask from him (bread). The Toisfos conclude: It is needed to be said that here to (in the case of travelers to do Mitzvah and in the case of Rav Chizda and Raba bar bar Huna traveling to visit the Leader of Exile) they are talking about a case like this one (i.e. these two cases of travelers are similar to the case of a lost object at the moment of taking care of it, in a way) that if they (the travelers) would bother themselves to fulfill the Mitzvah of Suka, they would be detracted from (their original) Mitzvahs. (i.e. if they look for a Sukah it, for example, may take longer for them to arrive to their destinations to fulfill the original Mitzvah. To summarize, it seems from this Toisfos that the principle Osek Be Mitzvah Potur min Ha Mitzvah, the one who is involved in one Mitzvah is exempt from another Mitzvah, applies only when the involvement with the second Mitzvah is detrimental in some way to the original Mitzvah).

7. One who is sent to do a mitzvah is exempt from the (Mitzvah of) Sukkah whether in the daytime or nighttime. Parentheses (Rema): And also see what I said in Siman 38.