Musnad Ahmad 299:4

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