Chapter on the Rulings on Forgetfulness (Ahkam al-Sahw)

Ahmad ibn Muhammad from al-Hasan ibn Ali ibn Faddal from Abu Jamilah from Zayd al-Shahham Abu Usamah, who said: I asked Imam عليه السلام about a man who prayed ‘Asr with six rak‘ahs or five rak‘ahs. Imam عليه السلام said: "If he is certain that he prayed five or six, then he must repeat it. But if he does not know whether he added or shortened, then he should say the takbir while sitting, then perform two rak‘ahs, reciting in them the Opening of the Book (Al-Fatihah), at the end of his prayer, then perform the tashahhud. And if he is certain that he prayed only two or three rak‘ahs, and then left and spoke without realizing that he had not completed the prayer, then he must complete what remains of the prayer. For indeed, the Prophet of Allah سبحانه وتعالى, peace be upon him and his family, once prayed two rak‘ahs with the people, then forgot until He صلى الله عليه وعلى آله وسلم finished. Dhu al-Shimalayn said to Him صلى الله عليه وعلى آله وسلم: 'O Messenger of Allah سبحانه وتعالى, has something happened in the prayer?' So He صلى الله عليه وعلى آله وسلم said: 'O people, did Dhu al-Shimalayn speak the truth?' They said: 'Yes, you only prayed two rak‘ahs.' So He صلى الله عليه وعلى آله وسلم stood and completed what remained of His صلى الله عليه وعلى آله وسلم prayer."