If anyone fears that he will not wake up at the end of the night, let him offer Witr prayer at the beginning of the night; and anyone expects to wake up at the end of the night, let him offer Witr prayer at the end of the night. Indeed, prayer at the end of the night is witnessed (by the angels) and that is better

Jābir (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "If anyone fears that he will not wake up at the end of the night, let him offer Witr prayer at the beginning of the night; and anyone expects to wake up at the end of the night, let him offer Witr prayer at the end of the night. Indeed, prayer at the end of the night is witnessed (by the angels) and that is better."