Musnad Ahmad 299:3

'Abd al-Samad narrated to us, he said: Hammad narrated to us, from al-Jurayri, from Abu al-'Ala', 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, and follow the weakest among them, and take a mu'adhdhin who does not take a wage for his adhan."