Musnad Ahmad 33:95

Waki’ narrated to us, Ibn Abi Layla narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever among you thinks that he will not wake up at the end of it, let him perform Witr at the beginning of it. And whoever among you thinks that he will wake up at the end of it, let him perform Witr at the end of it, for the prayer at the end of the night is witnessed, and it is better."