سنن ابن ماجه

Abu Mijlaz narrated that Ibn ‘Umar said: “The Messenger of Allah (ﷺ) said: ‘Night prayers are to be offered two by two, and Witr is one Rak’ah.’ I said: ‘What do you think if I become drowsy and I want to sleep?’ He said: ‘Put “what do you think” up there with that star? (i.e., don’t think about it at all).’ I raised my head and saw As- Simak.* He repeated that the Messenger of Allah (ﷺ) said, ‘Night prayers are to be offered two by two, and Witr is one Rak’ah, before dawn.’”
* Here, As-Simak refers to a star or stars, either Arcturus (As-Simak Ar-Ramih) or Spica, also called Alpha Virginis (As-Simak Al-A'zal).