Musnad Abu Ya'la al-Mawsili 109:107

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