Musannaf Ibn Abi Shayba 3:4498

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Hammad bin Zayd narrated to us, from Anas bin Sirin, that Umar used to recite the two *Mu'awwidhatayn* (Surah Al-Falaq and Surah An-Nas) in the *Witr* prayer.