Sahih Muslim Book - 5, Hadith - 1284

وَحَدَّثَنَاهُ عَوْنُ بْنُ سَلاَّمٍ الْكُوفِىُّ أَخْبَرَنَا أَبُو بَكْرٍ النَّهْشَلِىُّ عَنْ عَبْدِ الرَّحْمَنِ بْنِ الأَسْوَدِ عَنْ أَبِيهِ عَنْ عَبْدِ اللَّهِ قَالَ صَلَّى بِنَا رَسُولُ اللَّهِ -صلى الله عليه وسلم- خَمْسًا فَقُلْنَا يَا رَسُولَ اللَّهِ أَزِيدَ فِى الصَّلاَةِ قَالَ « وَمَا ذَاكَ ». قَالُوا صَلَّيْتَ خَمْسًا. قَالَ « إِنَّمَا أَنَا بَشَرٌ مِثْلُكُمْ أَذْكُرُ كَمَا تَذْكُرُونَ وَأَنْسَى كَمَا تَنْسَوْنَ ». ثُمَّ سَجَدَ سَجْدَتَىِ السَّهْوِ.

It was narrated that 'Abdullah said: "The Messenger of Allah (s.a.w) led us in prayer and prayed five (Rak'ah). We said: 'O Messenger of Allah, has something been added to the prayer?' He said: 'Why is that?' They said: 'You prayed five (Rak'ah).' He said: 'I am human like you. I remember as you remember and I forget as you forget.' Then he did the two prostrations of As-Sahw (forgetfulness).