'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.

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ سَعْدِ بْنِ عَمَّارِ بْنِ سَعْدٍ، مُؤَذِّنِ رَسُولِ اللَّهِ ـ ﷺ ـ حَدَّثَنِي أَبِي عَنْ أَبِيهِ عَنْ جَدِّهِ أَنَّ أَذَانَ بِلاَلٍ كَانَ مَثْنَى مَثْنَى وَإِقَامَتُهُ مُفْرَدَةً .