"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  

"At the time of the Messenger of Allah ﷺ the phrases of the Adhan were said twice and the phrases of the Iqamah were said once, except that you should say: 'Qad Qamatis-Salah, Qad Qamatis-Salah (prayer is about to begin, prayer is about to begin).'"  

كَانَ الأَذَانُ عَلَى عَهْدِ رَسُولِ اللَّهِ ﷺ مَثْنَى مَثْنَى وَالإِقَامَةُ مَرَّةً مَرَّةً إِلاَّ أَنَّكَ تَقُولُ قَدْ قَامَتِ الصَّلاَةُ قَدْ قَامَتِ الصَّلاَةُ