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
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
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.
أَنْ يَعْلَمُوا وَقْتَ الصَّلاَةِ بِشَىْءٍ يَعْرِفُونَهُ فَذَكَرُوا أَنْ يُورُوا نَارًا أَوْ يَضْرِبُوا نَاقُوسًا فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ
"The Messenger of Allah ﷺ commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once."
إِنَّ رَسُولَ اللَّهِ ﷺ أَمَرَ بِلاَلاً أَنْ يَشْفَعَ الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ
"The Messenger of Allah ﷺ commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once." (Using translation from Nasāʾī 627)
أَنَّ رَسُولَ اللهِ ﷺ «أَمَرَ بِلَالًا أَنْ يَشْفَعَ الْأَذَانَ وَأَنْ يُوتِرَ الْإِقَامَةَ»