Ahmad ibn Muhammad ibn Isa from Ali ibn al-Nu‘man from Sa‘id al-A‘raj, who said: I heard Abu Abdillah عليه السلام say: "The Messenger of Allah سبحانه وتعالى, peace be upon him and his family, once prayed and then performed the salam after two rak‘ahs. Those behind Him صلى الله عليه وعلى آله وسلم said: 'O Messenger of Allah سبحانه وتعالى, has something occurred in the prayer?' He صلى الله عليه وعلى آله وسلم said: 'What is that?' They said: 'You only prayed two rak‘ahs.' He صلى الله عليه وعلى آله وسلم said: 'Is that so, O Dhu al-Yadayn?', as he used to be called Dhu al-Shimalayn. He (Dhu al-Shimalayn) replied: 'Yes.' So the Prophet صلى الله عليه وعلى آله وسلم continued from where He صلى الله عليه وعلى آله وسلم had left off, completed the prayer as four rak‘ahs, and said: 'Indeed, Allah سبحانه وتعالى, the Mighty and Majestic, is the One عز وجل who made me forget, as a mercy to the ummah. Do you not see that if a man were to do this, he would be reproached, and it would be said to him: ‘Your prayer is not accepted?' But whoever experiences this now, let him know that the Messenger of Allah سبحانه وتعالى, peace be upon him and his family, has already established the precedent and it has become a model to follow.' And He صلى الله عليه وعلى آله وسلم performed two prostrations (sajdatayn) because of the speech that took place."