It is narrated from al-Hasan (al-Basri) (may Allah have mercy on him) that the Prophet (peace and blessings be upon him) said: "The muezzins are the trustees of the Muslims over their prayers." And he mentioned other things along with it. This report is mursal and is a supporting witness (shahid) for the previous report.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2000