Musnad Abu Ya'la al-Mawsili 108:138

On the authority of Jabir, I heard the Prophet (ﷺ) say: "Whoever 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 is eager to wake up at the end of the night, let him perform Witr at the end of the night, for the recitation is witnessed at the end of the night, and that is better."