The first Adhan used to be when the Imam sat on the Minbar on Friday, at the time of the Messenger of Allah ﷺ and Abu Bakr and 'Umar. During the caliphate of 'Uthman, when the number of people increased, 'Uthman commanded that a third Adhan be given on Friday, so that Adhan was given from the top of Az-Zawra, and that is how it remained.  

أَنَّ الأَذَانَ كَانَ أَوَّلُ حِينَ يَجْلِسُ الإِمَامُ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةَ فِي عَهْدِ رَسُولِ اللَّهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ فَلَمَّا كَانَ فِي خِلاَفَةِ عُثْمَانَ وَكَثُرَ النَّاسُ أَمَرَ عُثْمَانُ يَوْمَ الْجُمُعَةِ بِالأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ  

"The third Adhan was ordered by 'Uthman when the number of people in Al-Madinah increased. The Messenger of Allah ﷺ only had one Adhan, and the Adhan on Friday was when the Imam sat down."  

أَخْبَرَهُ قَالَ إِنَّمَا أَمَرَ بِالتَّأْذِينِ الثَّالِثِ عُثْمَانُ حِينَ كَثُرَ أَهْلُ الْمَدِينَةِ وَلَمْ يَكُنْ لِرَسُولِ اللَّهِ ﷺ غَيْرُ مُؤَذِّنٍ وَاحِدٍ وَكَانَ التَّأْذِينُ يَوْمَ الْجُمُعَةِ حِينَ يَجْلِسُ الإِمَامُ  

"Bilal used to call the Adhan when the Messenger of Allah ﷺ sat on the Minbar on Friday, and when he came down he would say the Iqamah. It continued like that during the time of Abu Bakr and 'Umar, may Allah (SWT) be pleased with them."  

كَانَ بِلاَلٌ يُؤَذِّنُ إِذَا جَلَسَ رَسُولُ اللَّهِ ﷺ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ فَإِذَا نَزَلَ أَقَامَ ثُمَّ كَانَ كَذَلِكَ فِي زَمَنِ أَبِي بَكْرٍ وَعُمَرَ ؓ