The reference for this is custom (al-'urf). This is the opinion of some of al-Shafi'i's companions. Al-Khiraqi said regarding the prostration of forgetfulness: He performs it as long as he is in the mosque, for it is the place of prayer, so the closeness or length of the interval is defined by it. [Some of the companions of al-Shafi'i said] (23): The long interval is the duration of a rak'ah. This is the text narrated from al-Shafi'i. Others said: It is the duration of the prayer in which he forgot the pillar (24). [Our argument is that] (25) there is no limit for it in the Sacred Law, so it is referred back to custom, [like everything else that has no fixed limit] (26).
Section: [And when the omitted part is the salutation, he performs it and nothing more; if it was the tashahhud, he performs it and says the salutation; if it was something other than these two, he performs a complete rak'ah, then recites the tashahhud, says the salutation, and performs the prostrations] (27) of forgetfulness. Al-Shafi'i said: He performs the pillar and what follows it, and nothing else. The discussion on this will follow in the chapter on the prostration of forgetfulness. Ahmad, may Allah have mercy on him, said [in the narration of al-Athram] (28), regarding one who forgot a prostration from the fourth rak'ah, then performed the salutation and spoke: If the speech he engaged in was related to the affairs of the prayer, he makes up a rak'ah and does not count the final rak'ah; for it is not completed except by its two prostrations, and since he did not perform the two prostrations with the rak'ah and proceeded to perform an action after the single prostration, he makes up a rak'ah, then recites the tashahhud, says the salutation, and performs the two prostrations of forgetfulness. But if he spoke about something unrelated to the affairs of the prayer, he must start the prayer anew. Abu 'Abd Allah said: They claim that Malik also held this view. Perhaps Ahmad, may Allah have mercy on him, based his view on the hadith of Dhu al-Yadayn, and that the Prophet, may Allah bless him and grant him peace, spoke and asked Abu Bakr and 'Umar: "Is what Dhu al-Yadayn says true?" Then he built upon what had passed of his prayer. And regarding...
(23) In (M): "The companions of al-Shafi'i differed; some said it is as we say, and others said."(24) In (M), after this: "And what we have said is more correct."(25) In (M): "Because."(26) In (M): "And it is not permissible to limit it by arbitrary opinion."(27) In (M): "And he is obligated to perform a rak'ah, unless the forgotten part is the tashahhud and the salutation, in which case he performs them, says the salutation, and then prostrates."(28) Omitted from the original.
