Musnad Abu Ya'la al-Mawsili 3:200

Khallad ibn Aslam narrated to us, al-Nadr ibn Shumayl narrated to us, Isra'il narrated to us, from Abu Ishaq, from al-Harith, from Ali, who said: The Messenger of Allah (peace be upon him) used to perform Witr with nine surahs. In the first rak'ah: "Alhakum al-takathur," "Inna anzalnahu fi laylat al-qadr," and "Idha zulzilat al-ardu." In the second: "Al-'asr," "Idha ja'a nasru Allahi wal-fath," and "Inna a'taynaka al-kawthar." In the third: "Qul ya ayyuha al-kafirun," "Tabbat," and "Qul huwa Allahu ahad."