Abdullah bin Rabah (may Allah have mercy on him) narrates from Sayyiduna Abu Qatadah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) asked Sayyiduna Abu Bakr (may Allah be pleased with him): "When do you pray witr?" He said: I pray witr before sleeping. He (peace and blessings be upon him) asked Sayyiduna Umar (may Allah be pleased with him): "When do you pray witr?" He said: I pray witr after sleeping and getting up. He (peace and blessings be upon him) said to Sayyiduna Abu Bakr (may Allah be pleased with him): "You have adopted caution," and said to Sayyiduna Umar (may Allah be pleased with him): "You have adopted resolve (azimah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4839