كتاب الصلاة
4  
The Book of Prayers
(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
(2)
باب الأَمْرِ بِشَفْعِ الأَذَانِ وَإِيتَارِ الإِقَامَةِ ‏
Sahih Muslim 378 a

Anas reported:

Bilal was commanded (by the Messenger of Allah) to repeat (the phrases of) Adhan twice and once in Iqama. The narrator said: I made a mention of it before Ayyub who said: Except for saying: Qamat-is-Salat [the time for prayer has come].
حَدَّثَنَا خَلَفُ بْنُ هِشَامٍ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، ح وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا إِسْمَاعِيلُ ابْنُ عُلَيَّةَ، جَمِيعًا عَنْ خَالِدٍ الْحَذَّاءِ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسٍ، قَالَ أُمِرَ بِلاَلٌ أَنْ يَشْفَعَ، الأَذَانَ وَيُوتِرَ الإِقَامَةَ ‏.‏ زَادَ يَحْيَى فِي حَدِيثِهِ عَنِ ابْنِ عُلَيَّةَ فَحَدَّثْتُ بِهِ أَيُّوبَ فَقَالَ إِلاَّ الإِقَامَةَ ‏.‏
Reference : Sahih Muslim 378a
In-book reference : Book 4, Hadith 2
USC-MSA web (English) reference : Book 4, Hadith 736
  (deprecated numbering scheme)