Sahih Bukhari Volume 1, Book 10, Hadith 714

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكِ بْنِ أَنَسٍ، عَنْ أَيُّوبَ بْنِ أَبِي تَمِيمَةَ السَّخْتِيَانِيِّ، عَنْ مُحَمَّدِ بْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم انْصَرَفَ مِنَ اثْنَتَيْنِ، فَقَالَ لَهُ ذُو الْيَدَيْنِ أَقَصُرَتِ الصَّلاَةُ أَمْ نَسِيتَ يَا رَسُولَ اللَّهِ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَصَدَقَ ذُو الْيَدَيْنِ ‏"‏‏.‏ فَقَالَ النَّاسُ نَعَمْ‏.‏ فَقَامَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَصَلَّى اثْنَتَيْنِ أُخْرَيَيْنِ ثُمَّ سَلَّمَ، ثُمَّ كَبَّرَ فَسَجَدَ مِثْلَ سُجُودِهِ أَوْ أَطْوَلَ‏

Narrated By Abu Huraira : Once Allah's Apostle prayed two Rakat (instead of four) and finished his prayer. Dhu-l-yadain asked him whether the prayer had been reduced or whether he had forgotten. Allah's Apostle asked the people whether Dhu-l-yadain was telling the truth. The people replied in the affirmative. Then Allah's Apostle stood up, offered the remaining two Rakat and then finished his prayer with Taslim and then said, "Allahu Akbar." He followed it with two prostrations like ordinary prostrations or a bit longer.