Is it recommended that the muezzin be sighted?
Chapter on Adhan
Al-Mughni
Book of Prayer
Primary text
It is recommended that the muezzin be sighted because a blind person may not know the correct time, leading to error. However, the call to prayer (Adhan) made by a blind person is valid. This is supported by the practice of Ibn Umm Maktum, who used to call the Adhan for the Prophet, peace be upon him. Ibn Amr narrated that Ibn Umm Maktum was blind and would not give the call until someone told him, "It has become morning." This hadith is narrated by Al-Bukhari.
Supporting text
It is recommended that a sighted person accompany the blind muezzin to inform him of the time, or that the blind person calls after a sighted muezzin, similar to how Ibn Umm Maktum used to call after Bilal.