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.‏