Musnad Ahmad 299:5

'Affan narrated to us, he said: Hammad bin Zayd narrated to us, Sa'id al-Jurayri informed us, from Abu al-'Ala', from Mutarrif, from 'Uthman bin Abi al-'As, who said: I said: "O Messenger of Allah, make me the imam of my people." He said: "You are their imam, and follow the weakest among them, and take a mu'adhdhin who does not take a wage for his adhan."