Musannaf Ibn Abi Shayba 22:265

Abu Mu'awiyah narrated to us, from al-A'mash, from Khaythama, from Abdullah ibn Amr, who said: I reached him while he was looking into the Mushaf. I said, "What are you reading in the Mushaf?" He said, "My hizb (portion) which I recite during the night."