In the lifetime of the Prophet, Abu Bakr and ʿUmar, the Adhan for the Jumua prayer used to be pronounced when the Imam sat on the pulpit. But during the Caliphate of ʿUthman when the Muslims increased in number, a third Adhan at Az-Zaura' was added. Abu ʿAbdullah said, "Az-Zaura' is a place in the market of Medina."  

كَانَ النِّدَاءُ يَوْمَ الْجُمُعَةِ أَوَّلُهُ إِذَا جَلَسَ الإِمَامُ عَلَى الْمِنْبَرِ عَلَى عَهْدِ النَّبِيِّ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ ؓ فَلَمَّا كَانَ عُثْمَانُ ؓ وَكَثُرَ النَّاسُ زَادَ النِّدَاءَ الثَّالِثَ عَلَى الزَّوْرَاءِ  

I heard As-Saib bin Yazid, saying, "In the lifetime of Messenger of Allah ﷺ, and Abu Bakr and ʿUmar, the Adhan for the Jumua prayer used to be pronounced after the Imam had taken his seat on the pulpit. But when the people increased in number during the caliphate of ʿUthman, he introduced a third Adhan (on Friday for the Jumua prayer) and it was pronounced at Az-Zaura' and that new state of affairs remained so in the succeeding years.  

سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ يَقُولُ إِنَّ الأَذَانَ يَوْمَ الْجُمُعَةِ كَانَ أَوَّلُهُ حِينَ يَجْلِسُ الإِمَامُ يَوْمَ الْجُمُعَةِ عَلَى الْمِنْبَرِ فِي عَهْدِ رَسُولِ اللَّهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ ؓ فَلَمَّا كَانَ فِي خِلاَفَةِ عُثْمَانَ ؓ وَكَثُرُوا أَمَرَ عُثْمَانُ يَوْمَ الْجُمُعَةِ بِالأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ  

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  

During the time of the Prophet ﷺ and Abu Bakr and 'Umar the call to the Friday prayer was first made at the time when the imam was seated on the pulpit (for giving the sermon). When the time of 'Uthman came, and the people became abundant, 'Uthman ordered to make a third call to the Friday prayer. It was made on al-Zaura' (a house in Medina). The rule of action continued to the same effect.  

أَنَّ الأَذَانَ كَانَ أَوَّلُهُ حِينَ يَجْلِسُ الإِمَامُ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ فِي عَهْدِ النَّبِيِّ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ ؓ فَلَمَّا كَانَ خِلاَفَةُ عُثْمَانَ وَكَثُرَ النَّاسُ أَمَرَ عُثْمَانُ يَوْمَ الْجُمُعَةِ بِالأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ  

During the time of the Prophet ﷺ and Abu Bakr and 'Umar the call to the Friday prayer was first made at the time when the imam was seated on the pulpit (for giving the sermon). When the time of 'Uthman came, and the people became abundant, 'Uthman ordered to make a third call to the Friday prayer. It was made on al-Zaura' (a house in Medina). The rule of action continued to the same effect. (Using translation from Abū Dāʾūd 1087)   

«أَنَّ الْأَذَانَ كَانَ أَوَّلُهُ حِينَ يَجْلِسُ الْإِمَامُ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ فِي عَهْدِ رَسُولِ اللهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ فَلَمَّا كَانَ خِلَافَةُ عُثْمَانَ كَثُرَ النَّاسُ وَأَمَرَ عُثْمَانُ بْنُ عَفَّانَ يَوْمَ الْجُمُعَةِ بِالْأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ»