Bulugh al-Maram 335

Narrated Ibn Mas'ud (RA): Allah's Messenger (ﷺ) offered prayer and when he said Taslim (salutation), he was asked, "O Allah's Messenger (ﷺ) ! Has something new happened to the Salat (prayer)?" He asked, "What is that?" They said, "You have prayed so many and so many (Rak'at)." He [Ibn Mas'ud (RA)] said, "He then bent his legs, faced the Qiblab, and made two prostrations and then said the Taslim (salutations). Then he faced us and said, "If something new is introduced to the Salat (prayer), I shall inform you but I am a human being like you, I forget just as you forget; so if I forget remind me and if any of you is in doubt about his Salat (prayer) he should act upon what he thinks is correct and complete his prayer in that respect and then he should make two prostrations." [Agreed upon].