Mukhtasar Bukhari

Narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that a man asked the Noble Prophet (peace and blessings of Allah be upon him) about the night prayer (Tahajjud). The Messenger of Allah (peace and blessings of Allah be upon him) said: "The night prayer is offered in pairs of two rak‘ahs. Then when one of you fears that dawn is approaching, he should pray one rak‘ah, which will make whatever he has prayed odd (witr) for him."