Sahih Al Bukhari Hadith Number 1097

Narrated ‘Amir bin Rabi`a: I saw the Prophet (peace be upon him) on his Mount praying Nawafil by nodding his head, whatever direction he faced, but Allah’s Messenger (peace be upon him) never did the same in offering the compulsory prayers.

18  Shortening the Prayers (At-Taqseer)
Hadith number 1097
Volume no 2
Chapter:9 (To get down to offer compulsory Salat)