Musnad Ahmad 30:1788

Musa ibn Dawud narrated to us, Zuhayr narrated to us, from Abu Ishaq, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The caller to prayer (mu'adhdhin) is a trustee, and the prayer leader (imam) is a guarantor. O Allah, guide the prayer leaders, and forgive the callers to prayer."