Hadith 213
Sayyiduna Ibn Umar (may Allah be pleased with them) reports that he asked the Messenger of Allah (peace and blessings be upon him) about the night prayer, so the Messenger of Allah (peace and blessings be upon him) said: "It is (offered) two by two rak‘ahs. When any of you fears that dawn is approaching, let him pray one rak‘ah as witr."