Sunan Abi Dawud Book 2, Hadith 144
The Messenger of Allah (ﷺ) said to Bilal: Do not call adhan until the dawn appears clearly to you in this way, stretching his hand in latitude.
Abu Dawud said: Shaddad did not see Bilal.
A mu'adhdhin of ‘Umar, named Masruh, called the Adhan for the morning prayer before the break of dawn; ‘Umar commanded him (to repeat). The narrator reported the tradition in a similar way.
Abu Dawud said: This tradition has been transmitted by al-Darawardi from ‘Ubaid Allah on the authority of Ibn ‘Umar, saying: there was a mu'adhdhin of ‘Umar, named Mas’ud. He then narrated the rest of the tradition. This version is more correct than one.
Ibn Umm Maktum was the mu'adhdhin of the Messenger of Allah (May peace be upon him) and he was blind.