Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1181
Abdullah reported: We prayed along with the Messenger of Allah (may peace he upon him) and he committed or omitted (something). Ibrahim said: By Allah, this is a misgiving of mine only. We said: Messenger of Allah, is there something new about the prayer? He (the Holy Prophet) said: No. We told him about what he had done. He (the Holy Prophet) said: When a man commits or omits (something in prayer), he should perform two prostrations, and he then himself performed two prostrations.