The Book of Prayers
4/2. Chapter:
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
Sahih Muslim
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 .
Anas reported: Bilal was commanded (by the Holy Prophet) to repeat the phrases twice in Adhan, and once in lqama.
Reference : Sahih Muslim 378dIn-book reference : Book 4, Hadith 5USC-MSA web (English) reference : Book 4, Hadith 739