Mukhtasar Bukhari

Narrated Abdullah bin Umar (may Allah be pleased with them): A man asked the Prophet (peace be upon him) while he was on the pulpit about the prayer at night. The Prophet (peace be upon him) said: "Pray two by two (rak'ahs)." Then when any one of you fears that morning is approaching, let him pray one rak'ah, and that one rak'ah will make what he has prayed odd (witr) for him. And Ibn Umar (may Allah be pleased with them) used to say: "Make your last prayer at night witr, because the Prophet (peace be upon him) ordered this."