Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that the Messenger of Allah (peace and blessings be upon him) used to pray witr with three rak'ahs: in the first he would recite "Sabbih isma rabbika al-a'la", in the second "Qul ya ayyuha al-kafirun", and in the third "Qul huwa Allahu ahad".
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12320