Sahih Muslim Book 4, Hadith Number 1502.
Chapter : Praying in houses when it rains.
Sa’id b. Yasar reported: I was travelling along with Ibn ‘Umar on the way to Mecca. Sa’id said: When I apprehended dawn, I dismounted (the ride) and observed Witr prayer and then again joined him. Ibn ‘Umar said to me: Where were you? I said: I apprehended the appearance of dawn, so I dismounted and observed Witr prayer. Upon this ‘Abdullah said: Is there not a model pattern for you in the Messenger of Allah (may peace be upon him)? I said: Yes, by Allah, and (then) he said: The Messenger of Allah (may peace be upon him) used to observe Witr prayer on the camel’s back.