Siman 105

It is stated in the Gemara (30b): How long should one wait between prayer and prayer? That is to say: for sometimes one must pray two prayers one after the other, such as a person who erred and wishes to pray a second time. And likewise between the shacharit prayer and the musaf prayer (Tosafot), when he does not say "Ashrei" in between (Tosafot of R"i), or if he forgot to pray and completes it during the time of the prayer that follows it, and prays two one after the other (Tur). How long should he wait between one and the other? One said: enough for his mind to be supplicated, so that his mind will be settled to arrange his words in the language of supplication (Rashi), as it is written: "And I supplicated [va'etchanan] the Lord" (Gemara). And one said: enough for his mind to be entreated, a language of "entreaty" as it is written: "And Moses entreated [vayechal]," see there.

The Rambam in the tenth chapter, law 15, ruled "so that his mind will be composed," see there. However, Rashi, of blessed memory, explained that they do not disagree, and they only disagree regarding the phrasing, see there. And the Tosafot wrote: "Regarding 'until it is entreated'" – it is explained in the Yerushalmi, that this is like the time it takes to walk four amot. Thus far their words. And in the Yerushalmi these names are not mentioned at all: "until it is composed," "until it is entreated." And thus it is stated there (fourth chapter, halacha 6), for Rabbi Yannai said: I saw Rabbi Yannai standing and praying..., and walking four amot and praying the Musaf prayer, see there. And from this wording it implies specifically that he must walk four amot. But the Tur and the Shulchan Aruch wrote: One who prays two prayers one after the other – must wait between one and the other the time it takes to walk four amot. Thus far his words. And the law is with them, for so it says there afterwards in the Yerushalmi. And not until he walks four amot, but even if he delayed the time it takes to walk four amot, see there. And if so, it requires investigation regarding the Tosafot, who did not write according to the conclusion of the Yerushalmi. (And it is also difficult: why did they note this "regarding 'until it is entreated'"? And perhaps they do not hold like Rashi that they do not disagree? And they disagree in this: that the one who says "so that his mind will be composed" – a delay of four amot is sufficient. And the one who says "until it is entreated" – specifically walking is required. And so wrote the Pnei Moshe there. And according to this, since the Rambam ruled "so that his mind will be composed" – the ruling of the Tur and the Shulchan Aruch stands. Examine and you will find it simple.)

And it is simple that the waiting is after he takes three steps backward. And one should not ask: if so, even without this he is compelled to wait the duration of four cubits until he returns to his place, as will be explained in siman 123? For here we are dealing with one who wishes to pray in the place where he stepped, and does not wish to return to his place (Magen Avraham). And the strictness is only regarding the Shmoneh Esrei prayers. But regarding other prayers, he may pray one after the other immediately, even several prayers (ibid.). However, according to our custom to say "Ashrei" between Shacharit and Musaf and the reading of the Torah—behold, there is a large interruption. And even the saying of "Ashrei" alone is more than the duration of walking four cubits.