"I asked Ibn 'Umar about the Adhan and he said: 'At the time of the Messenger of Allah ﷺ, the phrases of the Adhan were recited twice and the phrases of Iqamah once, except that you should say (the phrase) Qad qamat is-salah (prayer is about to begin)twice. When we heard 'prayer is about to begin' we would perform Wudu' and go out to pray.'" (Using translation from Nasāʾī 668)   

أَنَّهُ قَالَ كَانَ الْأَذَانُ عَلَى عَهْدِ رَسُولِ اللَّهِ ﷺ مَثْنَى مَثْنَى وَالْإِقَامَةُ مَرَّةً غَيْرَ أَنَّهُ كَانَ إِذَا قَالَ قَدْ قَامَتِ الصَّلَاةُ قَالَهَا مَرَّتَيْنِ فَإِذَا سَمِعْنَا الْإِقَامَةَ تَوَضَّأَ أَحَدُنَا وَخَرَجَ  

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

«أُمِرَ بِلَالٌ أَنْ يَشْفَعَ الْأَذَانَ وَيُوتِرَ الْإِقَامَةَ»  

Anas reported; Bilal was commanded to pronounce Adhan in double pairs and IQAMAH in single pairs. Hammam added in his version; “except IQAMAH”. (Using translation from Abū Dāʾūd 508)   

«أُمِرَ بِلَالٌ أَنْ يَشْفَعَ الْأَذَانَ وَيُوتِرَ الْإِقَامَةَ إِلَّا الْإِقَامَةَ»