Musannaf Ibn Abi Shayba 3:4451

6828. Abbad ibn al-Awwam narrated to us, from al-Ala’ ibn al-Musayyab, from his father, from Aisha, who said: Do not perform Witr with three [rak'ahs] that are "batra" (cut off/incomplete). Pray before it two rak'ahs or four.