Sayyiduna Abu Basrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "Allah, the Mighty and Majestic, has granted you an additional prayer, so guard it. He has set its time between Isha and Fajr, and it is the witr."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2155