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 b

Anas b. Malik reported:

They (the Companions) discussed that they should know the timings of prayer by means of something recognized by all. Some of them said that fire should be lighted or a bell should be rung. But Bilal was ordered to repeat the phrases twice in Adhan, and once in Iqama.
وَحَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ الْحَنْظَلِيُّ، أَخْبَرَنَا عَبْدُ الْوَهَّابِ الثَّقَفِيُّ، حَدَّثَنَا خَالِدٌ الْحَذَّاءُ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ ذَكَرُوا أَنْ يُعْلِمُوا، وَقْتَ الصَّلاَةِ بِشَىْءٍ يَعْرِفُونَهُ فَذَكَرُوا أَنْ يُنَوِّرُوا نَارًا أَوْ يَضْرِبُوا نَاقُوسًا فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَيُوتِرَ الإِقَامَةَ ‏.‏
Reference : Sahih Muslim 378b
In-book reference : Book 4, Hadith 3
USC-MSA web (English) reference : Book 4, Hadith 737
  (deprecated numbering scheme)