פירוש על ברכות 55:15

THE HALAKHA [IS THAT] HE RECITES THE AFTERNOON PRAYER [FIRST] AND THE ADDITIONAL PRAYER THEREAFTER. The Gemara cited a Braiso that teaches that when one has the two prayers, minchoh and musaf; he should say minchoh first because minchoh is constant as opposed to musaf which is said only from time to time. There is a general rule that when has to perform two mitzvos one does the constant one first. Tosfos gives us a practical application of this ruling. From here we learn that on Yom Kipur we should be careful to complete praying the shacharis prayer before six and a half hours of the day have gone by, which is before the time to pray minchoh, for if one is not careful to start musaf before six and a half hours we would be required to pray minchoh before musaf as is evident from the ruling of the Braiso.
This explanation of Tosfos assumes that when the Gemara speaks one who has two prayers before him, he must choose the more constant one; it refers to all situations where one has two prayers to recite. Tosfos will now show us that R’I has a different opinion on this issue. And R’I says that it is not necessary to pray the mincha prayers a six and a half hours on Yom Kipur before the musaf prayer. For that which was said in the Braiso that one must pray the minchoh prayer earlier, that is when one has to take care of his needs and he will not be able to pray the minchoh prayer at its time, usually after nine and a half hours, and he must therefore pray both prayers now. For example, he needs to attend a major banquet such as a wedding and he is concerned that he may extend his meal beyond the regular time to pray minchoh or that he might become intoxicated and be unable to pray even if he finishes the meal on time. However, if there is sufficient time to pray the minchoh prayer at its regular time after the musaf prayer, he need not precede the minchoh prayer before the musaf prayer; rather, he should pray in order musaf and then minchoh.

See above, fol. 27 a, p. 176.

See p. 176 n. 1.