Sahih Muslim Book - 5, Hadith - 1285

وَحَدَّثَنَا مِنْجَابُ بْنُ الْحَارِثِ التَّمِيمِىُّ أَخْبَرَنَا ابْنُ مُسْهِرٍ عَنِ الأَعْمَشِ عَنْ إِبْرَاهِيمَ عَنْ عَلْقَمَةَ عَنْ عَبْدِ اللَّهِ قَالَ صَلَّى رَسُولُ اللَّهِ -صلى الله عليه وسلم- فَزَادَ أَوْ نَقَصَ - قَالَ إِبْرَاهِيمُ وَالْوَهْمُ مِنِّى - فَقِيلَ يَا رَسُولَ اللَّهِ أَزِيدَ فِى الصَّلاَةِ شَىْءٌ فَقَالَ « إِنَّمَا أَنَا بَشَرٌ مِثْلُكُمْ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِىَ أَحَدُكُمْ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ ». ثُمَّ تَحَوَّلَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- فَسَجَدَ سَجْدَتَيْنِ.

It was narrated that 'Abdullah said: "The Messenger of Allah (s.a.w) offered Salat, and he added or omitted something" - Ibrahim (a narrator) said: "I am not sure." "It was said: 'O Messenger of Allah, has something been added to the prayer?' He said: 'I am human like you, and I forget as you forget. If one of you forgets something, let him prostrate twice while he is sitting.' Then the Messenger of Allah (s.a.w) turned around and prostrated twice.