Musnad Ahmad 30:2812

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