Musnad Ahmad 30:1849

Abdullah ibn Numayr narrated to us, from al-A’mash, who said: It was narrated to me from Abu Salih—and I do not see myself but that I heard it 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."