Musnad Ahmad 3385

It was narrated that Abu Mijlaz said: I asked Ibn ‘Umar about Witr and he said: I heard The Messenger of Allah (ﷺ) say: "One rak'ah at the end of the night." He said: And I asked 'Abdullah bin 'Abbas (رضي الله عنه) (about it) and he said: I heard the Messenger of Allah (ﷺ) say: "One rak'ah at the end of the night.”