Sahih Muslim Book 4, Hadith Number 1558.

Chapter : Excellence of observing two Rak’ahs in the mosque for one who comes back from a journey.

Abu Huraira reported. My friend (the Holy Prophet, may peace be upon him) has instructed me to do three things: three fasts during every month, two rak’ahs of the forenoon prayer, and observing Witr prayer before going to bed.