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

Al-‘Ayyashi, from Ja‘far bin Ahmad, who said: Ali bin al-Husayn and Ali bin Muhammad narrated to me, from al-‘Ubaydi, from Yunus, from al-‘Ala’, from Muhammad bin Muslim, from one of them (Imams عليه السلام), peace be upon them both, who said: He عليه السلام was asked about a man who joined the imam (of prayer) in prayer while the imam (of prayer) had already completed one rak‘ah before him. When the imam finished the prayer, the man left with the people, then remembered that he had missed a rak‘ah. Imam عليه السلام said: "He should repeat one rak‘ah, it is permissible for him to do so as long as he has not turned his face away from the qiblah. But if he has completely turned his face away, then he must start the prayer anew from the beginning."