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

Al-Husayn ibn Sa‘id from al-Hasan from Zur‘ah from Sama‘ah: Abu Abdillah عليه السلام, peace be upon him, said: "Whoever remembers his forgetfulness and completes the prayer, then there are no two prostrations of forgetfulness (sajdatay al-sahw) required upon him. Indeed, the Messenger of Allah سبحانه وتعالى, peace be upon him and his family, once led the people in Dhuhr prayer, prayed two rak‘ahs, then due to forgetfulness stopped. Dhu al-Shimalayn said to Him صلى الله عليه وعلى آله وسلم: 'O Messenger of Allah سبحانه وتعالى, has something new been revealed in the prayer?' He صلى الله عليه وعلى آله وسلم said: 'What is that?' He (Dhu al-Shimalayn) said: 'You only prayed two rak‘ahs.' The Messenger of Allah سبحانه وتعالى, peace be upon him and his family, said: 'Do you all say what he (Dhu al-Shimalayn) is saying?' They said: 'Yes.' So He صلى الله عليه وعلى آله وسلم stood up and completed the prayer with them and performed two prostrations of forgetfulness." He (Sama‘ah) said: “I asked, ‘What about someone who prays two rak‘ahs and thinks they were four, so he gives salam and leaves, then afterward remembers that he only prayed two rak‘ahs?’" Imam عليه السلام said: "He must begin the prayer from the start." I (Sama‘ah) asked: "Then why did the Messenger of Allah سبحانه وتعالى, peace be upon him and his family, not restart the prayer, and instead just completed what remained of it?" Imam عليه السلام said: "The Messenger of Allah سبحانه وتعالى, peace be upon him and his family, had not left His صلى الله عليه وعلى آله وسلم place. So if one has not left his place, then let him complete what is missing from his prayer, so long as he has retained the two initial rak‘ahs."