Al-Lu'lu Wal-Marjan 336 — The Book of Mosques and Places of Worship.

'Abdullah bin Mas'ud Narrated from:

The Prophet صلى الله عليه وسلم offered Salât (prayer) (and the subnarrator Ibrâhîm said, "I do not know whether he offered Salât more or less than usual"), and when he had finished As-Salât (the prayer) he was asked, "O Allâh's Messenger! Has there been any change in As-Salât ?" He said, "What is it?" The people said, "You have prayed so much and so much." So the Prophet صلى الله عليه وسلم bent his legs, faced the Qiblah (Ka'ba at Makka) and performed two prostrations (of Sahw) and finished his Salât with Taslîm (by turning his face to right and left saying: As-Salâmu-'Alaikum-wa rahmat ullâh).When he turned his face to us he said, "If there had been anything changed in As-Salât, surely I would have informed you, but I am a human being like you and liable to forget like you. So if I forget, remind me and if anyone of you is doubtful about his prayer, he should follow what he thinks to be correct and complete his Salât accordingly and finish it and perform two prostrations (of Sahw)." ( Sahîh Al-Bukhârî, Hadîth No. 394, Vol. 1)