Sahih Bukhari Volume 1, Book 8, Hadith 401

حَدَّثَنَا عُثْمَانُ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، قَالَ قَالَ عَبْدُ اللَّهِ صَلَّى النَّبِيُّ صلى الله عليه وسلم ـ قَالَ إِبْرَاهِيمُ لاَ أَدْرِي زَادَ أَوْ نَقَصَ ـ فَلَمَّا سَلَّمَ قِيلَ لَهُ يَا رَسُولَ اللَّهِ، أَحَدَثَ فِي الصَّلاَةِ شَىْءٌ قَالَ ‏"‏ وَمَا ذَاكَ ‏"‏‏.‏ قَالُوا صَلَّيْتَ كَذَا وَكَذَا‏.‏ فَثَنَى رِجْلَيْهِ وَاسْتَقْبَلَ الْقِبْلَةَ، وَسَجَدَ سَجْدَتَيْنِ ثُمَّ سَلَّمَ، فَلَمَّا أَقْبَلَ عَلَيْنَا بِوَجْهِهِ قَالَ ‏"‏ إِنَّهُ لَوْ حَدَثَ فِي الصَّلاَةِ شَىْءٌ لَنَبَّأْتُكُمْ بِهِ، وَلَكِنْ إِنَّمَا أَنَا بَشَرٌ مِثْلُكُمْ، أَنْسَى كَمَا تَنْسَوْنَ، فَإِذَا نَسِيتُ فَذَكِّرُونِي، وَإِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَحَرَّى الصَّوَابَ، فَلْيُتِمَّ عَلَيْهِ ثُمَّ يُسَلِّمْ، ثُمَّ يَسْجُدْ سَجْدَتَيْنِ ‏"

Narrated By 'Abdullah : The Prophet 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 Apostle! 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 bent his legs, faced the Qibla and performed two prostrations (of Sahu) and finished his prayers with Tasiim (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)."