Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1648
Abu Sa'id (al Khudri) reported Allah's Apostle (may peace be upon him) as saying: Observe Witr prayer before it is morning. Abu Sa'id reported that they (the Prophet's Companions) asked the Apostle of Allah (may peace be upon him) about Witr (prayer). (In reply to their inquiry) he said: Observe Witr prayer before it is morning.