Musnad Ahmad 33:628

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