Riyad us Saliheen: Hadith 1137

The Messenger of Allah (ﷺ) said, "He who apprehends that he may not get up in the later part of the night, should observe the Witr prayer in the first part of it; and he who is certain to get up in the last part of it, he should observe Witr at the end of the night, because Salat at the end of the night is attended (by the angels), and that is better."

[Muslim].