Sunan Ibn Majah Book 5, Hadith 1211

حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ مَنْصُورٍ، قَالَ شُعْبَةُ كَتَبَ إِلَىَّ وَقَرَأْتُهُ عَلَيْهِ قَالَ أَخْبَرَنِي إِبْرَاهِيمُ عَنْ عَلْقَمَةَ عَنْ عَبْدِ اللَّهِ قَالَ صَلَّى رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ صَلاَةً لاَ يَدْرِي أَزَادَ أَوْ نَقَصَ ‏.‏ فَسَأَلَ فَحَدَّثْنَاهُ فَثَنَى رِجْلَهُ وَاسْتَقْبَلَ الْقِبْلَةَ وَسَجَدَ سَجْدَتَيْنِ ثُمَّ سَلَّمَ ثُمَّ أَقْبَلَ عَلَيْنَا بِوَجْهِهِ فَقَالَ ‏ "‏ لَوْ حَدَثَ فِي الصَّلاَةِ شَىْءٌ لأَنْبَأْتُكُمُوهُ وَإِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِيتُ فَذَكِّرُونِي وَأَيُّكُمْ مَا شَكَّ فِي الصَّلاَةِ فَلْيَتَحَرَّ أَقْرَبَ ذَلِكَ مِنَ الصَّوَابِ فَيُتِمَّ عَلَيْهِ وَيُسَلِّمَ وَيَسْجُدَ سَجْدَتَيْنِ ‏"‏ ‏.‏

It was narrated that ‘Abdullah said:“The Messenger of Allah (ﷺ) offered prayer, and I am not sure whether he did something extra or omitted something. He asked, and we told him, so he turned to face the Qiblah and prostrated twice, then he said the Salam. Then he turned to face us and said: ‘If any new command has been revealed concerning the prayer, I would certainly have told you. But I am only human and I forget and you forget. If I forget, then remind me. And if anyone of you is uncertain about the prayer, let him do what is closest to what is correct, then complete the prayer, say the Salam and prostrate twice.”