Abu Sufyan narrates from Sayyiduna Jabir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever fears that he will not be able to wake up in the last part of the night, let him pray witr in the early part of the night and then sleep; and whoever is eager to get up in the last part of the night, let him pray 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 / 4837