ENOfficial
'Abdur-Rahman bin Sa'd bin 'Ammar bin Sa'd narrated (from his great-grandfather of the Messenger of Allah) that: In the Adhan of Bilal, the phrases were two by two, and in his Iqamah they were said once.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ سَعْدِ بْنِ عَمَّارِ بْنِ سَعْدٍ، مُؤَذِّنِ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ حَدَّثَنِي أَبِي عَنْ أَبِيهِ عَنْ جَدِّهِ أَنَّ أَذَانَ بِلاَلٍ كَانَ مَثْنَى مَثْنَى وَإِقَامَتُهُ مُفْرَدَةً .
sunnah.com