Abu al-Zubayr narrates from Sayyiduna Jabir (may Allah be pleased with him) that he said: I heard the Prophet (peace and blessings be upon him) say: "Whoever fears that he will not be able to wake up in the last part of the night, let him pray witr and then sleep; and whoever is firmly certain that he will wake up in the last part of the night, let him offer the witr in the last part of the night, for the recitation of the last part of the night is presented (before Allah), and that is better."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4838