Sahih Bukhari
Prayers (Salat)
Volume: 001 / Book Number: 008 / Hadith Number: 398
Narrated 'Abdullah:
"Once the Prophet offered five Rakat in Zuhr prayer. He was asked, "Is there an increase in the prayer?" The Prophet said, "And what is it?" They said, "You have prayed five Rakat.' So he bent his legs and performed two prostrations (of Sahu).