Sahih al-Bukhari 401

Narrated `Abdullah:The Prophet (ﷺ) prayed (and the sub-narrator Ibrahim said, "I do not know whether he prayed more or less than usual"), and when he had finished the prayers he was asked, "O Allah's Messenger (ﷺ)! Has there been any change in the prayers?" He said, "What is it?' The people said, "You have prayed so much and so much." So the Prophet (ﷺ) bent his legs, faced the Qibla and performed two prostration's (of Sahu) and finished his prayers with Taslim (by turning his face to right and left saying: 'As-Salamu `Alaikum- Warahmat-ullah'). When he turned his face to us he said, "If there had been anything changed in the prayer, 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 prayer accordingly and finish it and do two prostrations (of Sahu).