Hadith: 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

عن جابر رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «من خاف أن لا يقوم من آخِرِ الليل فليوتر أوله، ومن طَمِعَ أن يقوم آخره فليوتر آخر الليل، فإن صلاة آخر الليل مشهودة، وذلك أفضل».
[صحيح] - [رواه مسلم]

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."
Sahih/Authentic. - [Muslim]

Explanation

The Hadīth indicates that it is permissible for the Muslim to pray Witr at the beginning of the night, especially for the one who fears that he will not wake up at the end of the night. It also emphasizes the preference of offering Witr prayer at the end of the night because it is witnessed by the angels.