Musnad Ahmad 30:2968

Waki' narrated to us, Sufyan narrated to us, from al-A'mash, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Imam is a guarantor, and the Mu'adhdhin is a trustee. O Allah, guide the Imams, and forgive the Mu'adhdhins."