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).  

ذَكَرُوا النَّارَ وَالنَّاقُوسَ فَذَكَرُوا الْيَهُودَ وَالنَّصَارَى فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ  

The people mentioned the fire and the bell (as means proposed for announcing the time of prayer) and by such a suggestion they referred to the Jews and the Christians. But Bilal was ordered, "Pronounce the words of the Adhan (i.e. call for the prayer) twice and the Iqama once only."  

ذَكَرُوا النَّارَ وَالنَّاقُوسَ فَذَكَرُوا الْيَهُودَ وَالنَّصَارَى فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ 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  

"Bilal was commanded to say the phrases of the Adhan twice and the phrases of the Iqamah once." (Using translation from Ibn Mājah 730)  

أَنْ يَشْفَعَ الْأَذَانَ وَيُوتِرَ الْإِقَامَةَ