Sahih Bukhari Volume 9, Book 91, Hadith Number 355.

Narrated By ‘Abdullah : The Prophet led us in Zuhr prayer and prayer five Rakat. Somebody asked him whether the prayer had been increased.” He (the Prophet) said, “And what is that?” They (the people) replied, “You have prayed five Rakat.” Then the Prophet offered two prostrations (of Sahu) after he had finished his prayer with the Taslim.