The Book of the Adhan (The Call to Prayer)
7/2. Chapter:
Saying The Phrases of The Adhan Twice
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا عَبْدُ الْوَهَّابِ، عَنْ أَيُّوبَ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسٍ، قَالَ إِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَمَرَ بِلاَلاً أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ .
It was narrated that Anas said: "The Messenger of Allah (S.A.W) commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once."
Reference : Sunan an-Nasa'i 627In-book reference : Book 7, Hadith 2English translation : Vol. 1, Book 7, Hadith 628