Ibrahim narrates: I asked Alqamah - who used to spend the night with Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) - when Sayyiduna Abdullah (may Allah be pleased with him) used to pray witr. He said: "He would pray witr when as much of the night remained as had passed since the time of the Maghrib prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9371