Musnad Ahmad 572:10

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, from Sa’id al-Jurayri, from Abu al-‘Ala’, from Mutarrif ibn ‘Abd Allah, that ‘Uthman ibn Abi al-‘As said: "O Messenger of Allah, appoint me as the imam of my people." He said: "Follow the weakest among them, and take a mu'adhdhin who does not take a wage for his adhan."