Musnad Ahmad 33:509

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, from Jabir, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever among you fears that he will not wake up at night, let him pray Witr then sleep. And whoever among you is eager to wake up at night, let him pray Witr at the end of the night, for the recitation at the end of the night is witnessed, and that is better."