Abu Abd al-Rahman said: Sayyiduna Ali (may Allah be pleased with him) came out at the time Ibn al-Nabbah called the adhan, and recited: "By the night as it departs, and by the morning as it breathes" [Surah al-Takwir: 17-18]. Then he said: Where is the one who was asking about witr? This is the best time to pray witr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4525