Sahih Muslim 378
Sahih Muslim · Book 4: The Book of Prayers · The Command to say the phrases of the Adhan twice and the phrases of the iqamah once, except the phrase, "Prayer is about to begin," which is to be said twice
Anas reported:
وَحَدَّثَنِي عُبَيْدُ اللَّهِ بْنُ عُمَرَ الْقَوَارِيرِيُّ ، حَدَّثَنَا عَبْدُ الْوَارِثِ بْنُ سَعِيدٍ ، وَعَبْدُ الْوَهَّابِ بْنُ عَبْدِ الْمَجِيدِ ، قَالاَ حَدَّثَنَا أَيُّوبُ ، عَنْ أَبِي قِلاَبَةَ ، عَنْ أَنَسٍ ، قَالَ أُمِرَ بِلاَلٌ أَنْ يَشْفَعَ، الأَذَانَ وَيُوتِرَ الإِقَامَةَ .
Bilal was commanded (by the Holy Prophet) to repeat the phrases twice in Adhan, and once in lqama.