Sayyiduna al-Hajjaj bin Amr bin Ghaziyyah (may Allah be pleased with him), who was a Companion of the Messenger of Allah (peace and blessings be upon him), said: One of you considers it sufficient that he gets up at night and keeps praying until morning, and thinks that he has prayed tahajjud; whereas tahajjud is that a man prays after a sleep, then prays after another sleep, and this was the manner of prayer of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3203