Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): "When do you pray witr?" He said: I pray witr and then sleep. He (peace and blessings be upon him) said: "You have adopted caution." And he asked Sayyiduna Umar (may Allah be pleased with him): "When do you pray witr?" He said: I get up after sleeping and then pray witr. He (peace and blessings be upon him) said: "You have done the deed of the strong."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4841