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

Muhammad ibn Ahmad ibn Yahya from ‘Abbad ibn Sulayman from Sa‘d ibn Sa‘d from Muhammad ibn al-Qasim ibn al-Fudayl ibn Yasar from al-Hasan ibn al-Jahm, who said: I asked Abu al-Hasan عليه السلام, peace be upon him, about a man who prayed Dhuhr or ‘Asr and experienced an invalidating event (hadath) while sitting in the fourth rak‘ah. Imam عليه السلام said: "If he had already said “Ashhadu an la ilaha illa Allah wa anna Muhammada Rasulullah” (I bear witness that there is no god but Allah سبحانه وتعالى and that Muhammad صلى الله عليه وعلى آله وسلم is the Messenger of Allah سبحانه وتعالى), then he does not need to repeat the prayer. But if he had not yet recited the tashahhud before the invalidating event occurred, then he must repeat the prayer."