كتاب الأذان
Call to Prayers (Adhaan)
Chapter: Pronouncing the wording of Adhan for Salat (prayers) twice (in doubles)
باب الأَذَانُ مَثْنَى مَثْنَى
Sahih al-Bukhari 606

Narrated Anas bin Malik:

When the number of Muslims increased they discussed the question as to how to know the time for the prayer by some familiar means. Some suggested that a fire be lit (at the time of the prayer) and others put forward the proposal to ring the bell. Bilal was ordered to pronounce the wording of Adhan twice and of the Iqama once only.

حَدَّثَنَا مُحَمَّدٌ، قَالَ أَخْبَرَنَا عَبْدُ الْوَهَّابِ، قَالَ أَخْبَرَنَا خَالِدٌ الْحَذَّاءُ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ لَمَّا كَثُرَ النَّاسُ قَالَ ـ ذَكَرُوا ـ أَنْ يَعْلَمُوا وَقْتَ الصَّلاَةِ بِشَىْءٍ يَعْرِفُونَهُ، فَذَكَرُوا أَنْ يُورُوا نَارًا أَوْ يَضْرِبُوا نَاقُوسًا، فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ‏.‏
Reference : Sahih al-Bukhari 606
In-book reference : Book 10, Hadith 4
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 580
  (deprecated numbering scheme)