Musnad Ahmad 33:1062

He said: And the Messenger of Allah, may Allah bless him and grant him peace, 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."