Musannaf Ibn Abi Shayba 33:598

'Affan narrated to us, he said: Wuhayb narrated to us, he said: Dawud ibn Abi Hind narrated to us, from Abu Harb ibn Abi al-Aswad, from his father, who said: Abu Musa gathered the reciters and said: "Let no one enter upon you except those who have gathered (memorized) the Quran." He said: So there entered upon us about three hundred men. He exhorted us and said: "You are the reciters of this city and you are its elite. Do not let the time grow long upon you, lest your hearts harden as the hearts of the People of the Book hardened."