Language:
كتاب السهو
22  
Forgetfulness in Prayer
(2)
Chapter: If one offers five Rak'a (instead of four)
(2)
باب إِذَا صَلَّى خَمْسًا
Sahih al-Bukhari 1226

Narrated' `Abdullah:

Once Allah's Messenger (ﷺ) offered five rak`at in the Zuhr prayer, and somebody asked him whether there was some increase in the prayer. Allah's Messenger (ﷺ) said, "What is that?" He said, "You have offered five rak`at." So Allah's Messenger (ﷺ) performed two prostrations of Sahu after Taslim.

حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ، عَنِ الْحَكَمِ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم صَلَّى الظُّهْرَ خَمْسًا فَقِيلَ لَهُ أَزِيدَ فِي الصَّلاَةِ فَقَالَ ‏ "‏ وَمَا ذَاكَ ‏"‏‏.‏ قَالَ صَلَّيْتَ خَمْسًا‏.‏ فَسَجَدَ سَجْدَتَيْنِ بَعْدَ مَا سَلَّمَ‏.‏
Reference : Sahih al-Bukhari 1226
In-book reference : Book 22, Hadith 3
USC-MSA web (English) reference : Vol. 2, Book 22, Hadith 317
  (deprecated numbering scheme)