Musnad Ahmad 33:266

Abu Mu'awiyah and Muhammad ibn 'Ubayd narrated to us, they said: Al-A'mash narrated to us, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever among you fears that he will not wake up at the end of the night, let him perform Witr at the beginning of the night, then let him sleep. And whoever among you hopes to wake up at the end of the night, let him perform Witr at the end of the night, for the recitation of the end of the night is witnessed, and that is better."