Sahih Al Bukhari (401)

قال عبدالله: صلى النبي صلى الله عليه وسلم - قال إبراهيم: لا أدري - زاد أو نقص، فلما سلم قيل له: يا رسول الله، أحدث في الصلاة شيء؟ قال: (وما ذاك). قالوا: صليت كذا وكذا، فثنى رجليه، واستقبل القبلة، وسجد سجدتين، ثم سلم. فلما أقبل علينا بوجهه قال: (إنه لو حدث في الصلاة شيء لنبأتكم به، ولكن، إنما أنا بشر مثلكم، أنسى كما تنسون، فإذا نسيت فذكروني، وإذا شك أحدكم في صلاته، فليتحر الصواب فليتم عليه، ثم ليسلم، ثم يسجد سجدتين).

`Abdullah (may Allah be pleased with him) narrated: The Prophet (peace be upon him) 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 (peace be upon him)! 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 (peace be upon him) bent his legs, faced the Qiblah and performed two prostrations (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).”