They mentioned the fire and the bell, but then they brought up the Jews and the Christians, so Bilal was ordered to make the call to prayer in pairs and the iqamah in single phrases.
ذكروا النار والناقوس، فذكروا اليهود والنصارى، فأمر بلال أن يشفع الأذان، وأن يوتر الإقامة.
Bukhari related it in the chapter on the adhan (603), and Muslim in the chapter on prayer (378), both of them through the route of Abdul-Wahhab Thaqafi, who said, Khalid Haddan narrated to us, from Abu Qilaba, from Anas, and he related it. The wording given is Bukhari's. In a wording found in both of them, they mentioned that they should signal the time of prayer with something they would recognize, so they mentioned that they should kindle a fire, and in Muslim's version, that they should light a fire, or strike a bell. So he commanded Bilal to say the adhan in pairs and the iqamah singly.In one narration, "that they should kindle a fire." His saying, "that they should kindle a fire," means that they should light it and set it ablaze.