Sahih Ibn Khuzaymah

Sayyiduna Abu Sa'id Khudri رضي الله عنه narrates from the Messenger of Allah صلى الله عليه وسلم that he صلى الله عليه وسلم said: "When any one of you is uncertain in his prayer and does not know how many rak'ahs he has prayed—one, two, three, or four—then he should complete the number of rak'ahs about which he is in doubt, then while sitting in tashahhud, he should perform two prostrations. Thus, if his prayer was incomplete, he has now completed it, and the two prostrations will be a means of humiliation and disgrace for Satan. And if he had already completed his prayer, then this (extra) rak'ah and the two prostrations will be counted as voluntary (nafl) for him."

Imam Sahib says that Imam Rabi’ once narrated to us from his book, saying: Therefore, he should act on certainty, then perform two prostrations before saying the salam. And Abu Musa Durti and Yunus said in their narration that when any of you has doubt in his prayer, and he does not know whether he has prayed three rak‘ahs or four, then he should pray one rak‘ah and perform two prostrations before saying the salam. The rest of their narration is similar to the narration of Rabi’. Imam Abu Bakr, may Allah have mercy on him, said: In my view, this hadith is evidence that if a wealthy person’s wealth is not present with him (for example, he is sitting in another area for business, etc.), then he may separate the zakat from it and give it to those entitled to charity, with the intention that if his wealth remains safe, then this is its zakat, and if his wealth has been destroyed or lost, then this is voluntary charity. Then, if he receives correct information that his wealth is safe, then the wealth he gave to the entitled recipients as obligatory zakat for his absent wealth will be valid and correct. Because the Noble Prophet, peace and blessings be upon him, has declared the rak‘ah of the one praying to be valid, which he performed with two kinds of intention: that if his performed prayer was three rak‘ahs, then this would be his fourth obligatory rak‘ah, and if his prayer was already complete, then this rak‘ah would be nafl (supererogatory). And this rak‘ah will suffice for his obligatory prayer, even though he performed this rak‘ah with the intention of either obligatory or nafl.