Merits of the Helpers in Madinah (Ansaar)
Bukhari :: Book 5 :: Volume 58 :: Hadith 153
Narrated Masruq:
'Abdullah bin Masud was mentioned before Abdullah bin 'Amr who said, "That is a man I still love, as I heard the Prophet saying 'Learn the recitation of Quran from four from 'Abdullah bin Mas'ud --he started with him--Salim, the freed slave of Abu Hudaifa, Mu'adh bin Jabal and Ubai bin Ka'b."