Praise be to Allah, the Lord of the Worlds.
In the lifetime of the Prophet (PBUH), the Adhaan for Friday prayer used to be pronounced after the Imam sat on the pulpit, and there is no disagreement about the legality of this. Ibn Qhod`damah said: "There is no disagreement about the legality of the Adhaan for Friday prayer pronounced after the Imam sat on the pulpit, as it used to be at the time of the Prophet (PBUH)." As-Saa`b Ibn Yazeed said: "In the lifetime of the Prophet, Abu Bakr and `Umar, the Adhan for the Jumu`a prayer used to be pronounced when the Imam say on the pulpit." (Related by Bukhari). quoted from [Al-Mughni vol.1/pp.76].
As regards pronouncing the Adhaan before the Imam sat on the pulpit, it is the Sunnah of Uthman Bin Affaan and it was approved by the Prophet`s companions, and this proves its legality. The wisdom behind this Adhaan is to remind the people to get ready for the Friday prayer, one hour before its due time. As a result, this Adhaan remained until present, following the Sunnah of the Rightly Guided Caliphs (May Allah be pleased with them). The Messenger (PBUH) said: " So you must keep to my Sunnah and to the Sunnah of the Khulafa ar-Rashideen (the Rightly Guided Caliphs), those who guide to the right way. Cling to it stubbornly [literally: with your molar teeth]." {Musnad Ahmad}.
Nowadays, although the time between the two Adhaans for Friday prayer is short, it fulfills the essential purpose observed in the Sunnah of the Caliph Uthman Bin Affaan, giving prayer performers extra minutes to reach the mosque before the Imam sits on the Pulpit. There is no sound reason for denying this Adhaan (the one pronounced before the Imam sits on the pulpit), nowadays, as it fulfills the objective of the Sunnah of Uthman, and its legality is based on the companions` approval. This is despite the fact that some traditions describe this Adhaan as a newly invented matter (in religion), as reported by Ibn Rajab in his book [Fateh Al-Bari,vol.8/218-220].
It is considered newly invented (Mohdath) in the sense that it wasn`t pronounced in the lifetime of the Prophet (PBUH), and it wasn`t intended as prohibition. In fact, the companions invented and sanctioned it, and there is no Sharia-approved evidence that it is prohibited. Shafie scholars prefer one Adhaan for Jumu`ah, but none of them said that the Adhaan added by Caliph Uthman is prohibited or disliked. Al-Shafiee' (May Allah have mercy on his soul) said: "Regardless of the Adhaan at the time of Caliphs Uthman and Mu`awiah, I prefer one Adhaan for Jumu`ah, as was observed in the lifetime of the Prophet (PBUH)."
The prominent scholar Ibn Hajar Al-Haithami said: "The first Adhaan was invented by Uthman-it was also said by Muawiah-when the number of the people of Madinah increased. Then, the Sunnah of the Prophet (PBUH), one Adhaan, was followed, unless there was need for that first Adhaan. For example, going to the mosque for Jumu`ah prayer depended on hearing the first Adhaan delivered through the minarets." {Tohfat Al-Mohtaaj, vol.2/pp.460}. Al-Bakhri commented on Ibn Hajar: "His saying "Unless there was a need", indicates that one Adhaan doesn`t suffice." He added, "
As for the first Adhaan, none of the scholars had said that it is a Sunnah. Rather, they said that it was invented by Caliph Uthman when the number of the people of Madinah increased, and this indicates that this Adhaan is lawful and not a Sunnah." {E`Anat At-Talibeen Ala Hal AlFaad Fateh Al-Mo`eein,vol.2/pp.269}. And Allah the Almighty Knows Best.