Musannaf Ibn Abi Shayba 3:6186

Waki’ narrated to us, he said: Sufyan narrated to us, he said: A slave-woman of al-Rabi’ ibn Khuthaym narrated to me, she said: Al-Rabi’ used to read from the Mushaf (copy of the Quran), and if a person entered upon him, he would cover it.