Musnad Ahmad 30:2305

Qutaybah narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from Suhayl, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The Imam is a guarantor, and the Mu'adhdhin is a trustee. So may Allah guide the Imams, and forgive the Mu'adhdhins."