Chapter on Rulings on Forgetfulness (Sahw) in Prayer and What Requires Repeating of Prayer

Muhammad bin Ya‘qub, from Ali bin Ibrahim, from Muhammad bin ‘Isa, from Yunus, from Ibn Maskan, from Ibn Abi Ya‘fur, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who does not know whether he prayed two rak‘ahs or four. Imam عليه السلام said: "He should recite the tashahhud and give the taslim, then stand and pray two rak‘ahs and four prostrations, reciting the Opening of the Book (Surah al-Fatiha) in both of them, then recite the tashahhud and give the taslim. If he had already prayed four, then these two are (counted as) recommended prayers (nafilah), and if he had only prayed two, then these complete the four. And if he spoke (during this state), then he must perform the two prostrations of forgetfulness (sajdatay al-sahw)."