Musannaf Ibn Abi Shayba 3:934

Abu al-Ahwas narrated to us, from Mughirah, from Ibrahim, who said: The nephew of al-Aswad was their mu'adhdhin. He used to hasten the 'Asr prayer. So al-Aswad said to him: "You shall obey us regarding our adhan, or else you shall withdraw from being our mu'adhdhin."