Language:
كتاب الأذان
10
Call to Prayers (Adhaan)
(1)
Chapter: How the Adhan for Salat (Prayer) was started
(1)
باب بَدْءُ الأَذَانِ
وَقَوْلُهُ عَزَّ وَجَلَّ: {وَإِذَا نَادَيْتُمْ إِلَى الصَّلاَةِ اتَّخَذُوهَا هُزُوًا وَلَعِبًا ذَلِكَ بِأَنَّهُمْ قَوْمٌ لاَ يَعْقِلُونَ} وَقَوْلُهُ: {إِذَا نُودِيَ لِلصَّلاَةِ مِنْ يَوْمِ الْجُمُعَةِ}.
Sahih al-Bukhari 603

Narrated Anas:

The people mentioned the fire and the bell (they suggested those as signals to indicate the starting of prayers), and by that they mentioned the Jews and the Christians. Then Bilal was ordered to pronounce Adhan for the prayer by saying its wordings twice, and for the Iqama (the call for the actual standing for the prayers in rows) by saying its wordings once. (Iqama is pronounced when the people are ready for the prayer).

حَدَّثَنَا عِمْرَانُ بْنُ مَيْسَرَةَ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا خَالِدٌ الْحَذَّاءُ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسٍ، قَالَ ذَكَرُوا النَّارَ وَالنَّاقُوسَ، فَذَكَرُوا الْيَهُودَ وَالنَّصَارَى، فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ‏.‏
Reference : Sahih al-Bukhari 603
In-book reference : Book 10, Hadith 1
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 577
  (deprecated numbering scheme)