Musnad Ahmad 28:319

Waki' narrated to us, al-A'mash narrated to us, from Abu Wa'il, from Masruq, who said: We used to come to Abdullah ibn Amr and speak in his presence. One day, we mentioned Abdullah ibn Mas'ud, and he said: "You have mentioned a man whom I will continue to love ever since I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Take the Quran from four: from Ibn Umm Abd'—and he began with him—'and Mu'adh ibn Jabal, and Ubayy ibn Ka'b, and Salim, the freed slave of Abu Hudhayfah.'"