Musannaf Ibn Abi Shayba 22:296

Yahya ibn Abi Bukayr narrated to us, from Ibrahim ibn Rafi’, he said: I heard Sulayman, the freed slave of Umm Ali, that Mujahid used to dislike reciting the *Mu’awwidhat* (Surah al-Falaq and Surah an-Nas) alone, until he would place a surah with them.