Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) said: "The night prayer is offered two by two, and the witr is one rak'ah in the last part of the night."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Layth bin Abi Sulaym, who is thiqah (trustworthy), but he is a mudallis (one who practised tadlis).