Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1561
Abu Murra, the freed slave of Umm Hani, narrated on the authority of Abu Darda': My Friend (may peace be upon him) instructed me in three (acts), and I would never abandon them as long as I live. (And these three things are): Three fasts during every month, the forenoon prayer, and this that I should not sleep till I have observed the Witr prayer.