"They looked for something by means of which they could call out informing of (the time of) the prayer. Then Bilal was commanded to say the phrases of the Adhan twice and the phrases of the Iqamah once."  

الْتَمَسُوا شَيْئًا يُؤْذِنُونَ بِهِ عِلْمًا لِلصَّلاَةِ 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."  

أَنْ يَشْفَعَ الأَذَانَ وَيُوتِرَ الإِقَامَةَ  

In the Adhan of Bilal, the phrases were two by two, and in his Iqamah they were said once.  

أَنَّ أَذَانَ بِلاَلٍ كَانَ مَثْنَى مَثْنَى وَإِقَامَتُهُ مُفْرَدَةً  

"I saw Bilal calling the Adhan in fron of Messenger of Allah, (saying the phrases) two by two, and saying each phrase once in the Iqamah."  

حَدَّثَنِي مَعْمَرُ بْنُ مُحَمَّدِ بْنِ عُبَيْدِ اللَّهِ بْنِ أَبِي رَافِعٍ مَوْلَى النَّبِيِّ ﷺ حَدَّثَنِي أَبِي مُحَمَّدُ بْنُ عُبَيْدِ اللَّهِ عَنْ أَبِيهِ عُبَيْدِ اللَّهِ عَنْ أَبِي رَافِعٍ قَالَ رَأَيْتُ بِلاَلاً يُؤَذِّنُ بَيْنَ يَدَىْ رَسُولِ اللَّهِ ﷺ مَثْنَى مَثْنَى وَيُقِيمُ وَاحِدَةً