Sahih Bukhari Volume 2, Book 22, Hadith Number 317.

Narated By ‘Abdullah : Once Allah’s Apostle offered five Rakat in the Zuhr prayer, and somebody asked him whether there was some increase in the prayer. Allah’s Apostle said, “What is that?” He said, “You have offered five Rakat.” So Allah’s Apostle performed two prostrations of Sahu after Taslim.