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].  

أَنْ يَشْفَعَ الأَذَانَ وَيُوتِرَ الإِقَامَةَ زَادَ يَحْيَى فِي حَدِيثِهِ عَنِ ابْنِ عُلَيَّةَ فَحَدَّثْتُ بِهِ أَيُّوبَ فَقَالَ إِلاَّ الإِقَامَةَ  

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.  

أَنْ يُعْلِمُوا وَقْتَ الصَّلاَةِ بِشَىْءٍ يَعْرِفُونَهُ فَذَكَرُوا أَنْ يُنَوِّرُوا نَارًا أَوْ يَضْرِبُوا نَاقُوسًا فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَيُوتِرَ الإِقَامَةَ  

When the majority of the people discussed they should know, like the hadith narrated by al-Thaqafi (mentioned above) except for the words:" They (the people) should kindle fire."  

حَدَّثَنَا خَالِدٌ الْحَذَّاءُ بِهَذَا الإِسْنَادِ لَمَّا كَثُرَ النَّاسُ ذَكَرُوا أَنْ يُعْلِمُوا بِمِثْلِ حَدِيثِ الثَّقَفِيِّ غَيْرَ أَنَّهُ قَالَ أَنْ يُورُوا نَارًا  

Bilal was commanded (by the Holy Prophet) to repeat the phrases twice in Adhan, and once in lqama.  

أَنْ يَشْفَعَ الأَذَانَ وَيُوتِرَ الإِقَامَةَ