Musnad Abu Ya'la al-Mawsili 109:280

Ibn Numayr narrated to us, Muhammad ibn Ubayd and Ya'la 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 among you fears that he will not pray Witr at the end of the night, let him pray Witr at its beginning, for the recitation at the end of the night is witnessed, and it is better."