Hadith 2620
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Prophet Muhammad (peace and blessings be upon him) was asked, "What do you command us regarding how we should perform prayer at night?" The Prophet Muhammad (peace and blessings be upon him) said: "You should perform the prayer two by two. When you fear that dawn is near, then perform one rak'ah as witr."