كتاب الصلاة
4
The Book of Prayers
(4)
Chapter: It Is Recommended To Have Two Mu'adhdhin In A Single Masjid
(4)
باب اسْتِحْبَابِ اتِّخَاذِ مُؤَذِّنَيْنِ لِلْمَسْجِدِ الْوَاحِدِ ‏‏
Sahih Muslim 380 a

Ibn Umar reported:

The Messenger of Allah (ﷺ) had two Mu'adhdhins, Bilal and 'Abdullah b. Umm Maktum, who (latter) was blind.
حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم مُؤَذِّنَانِ بِلاَلٌ وَابْنُ أُمِّ مَكْتُومٍ الأَعْمَى ‏.‏
Reference : Sahih Muslim 380 a
In-book reference : Book 4, Hadith 7
USC-MSA web (English) reference : Book 4, Hadith 741
  (deprecated numbering scheme)