Mukhtasar Bukhari (Abridged) 600 — Long standing in Tahajjud is Sunnah

Narrated Abdullah bin Mas'ud (may Allah be pleased with him): I prayed the night prayer (Tahajjud) with the Prophet (peace and blessings of Allah be upon him) one night, and he kept standing (in prayer) until I intended to do something bad. He was asked: What did you intend? He replied: I intended to sit down and leave the Prophet (peace and blessings of Allah be upon him) standing.