Musnad Ahmad 28:47

Ya'la narrated to us, Al-A'mash narrated to us, from Abu Wa'il, from Masruq, who said: I was sitting with Abdullah ibn Amr, and he mentioned Abdullah ibn Mas'ud. He said: "That is a man whom I will never cease to love forever. I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Take the Quran from four: from Ibn Umm 'Abd'"—and he began with him—"and from Mu'adh, and from Salim, the freed slave of Abu Hudhayfah." Ya'la said: And I forgot the fourth.