Musnad Ahmad 5:117

Muhammad ibn Abdullah ibn al-Zubayr and Aswad ibn Amir narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from al-Harith, from Ali, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to perform Witr with nine surahs from the Mufassal." Aswad said: "He would recite in the first rak'ah: 'Alhakum al-takathur' (102), 'Inna anzalnahu fi laylat al-qadr' (97), and 'Idha zulzilat al-ard' (99). And in the second rak'ah: 'Wal-'asr' (103), 'Idha ja'a nasrullahi wal-fath' (110), and 'Inna a'taynaka al-kawthar' (108). And in the third rak'ah: 'Qul ya ayyuha al-kafirun' (109), 'Tabbat yada abi lahab' (111), and 'Qul huwallahu ahad' (112)."