In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- When the people’s numbers in Madinah increased at the time of the Rightly Guided Caliph `Uthman ibn`Affan (may Allah be pleased with him), he decided to add another adhan.
Answering your question, the late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
At the time of the Prophet (peace be upon him), there was one adhan, with the iqamah. When the Prophet came out to deliver the khutbah and lead the prayer, the mu’adhdhin gave the adhan, then the Prophet delivered the two khutbahs, then the iqamah would be given. This is what is known and this is what is narrated in the books of Sunnah and it is something well-known to people of knowledge and faith.
But then the people’s numbers in Madinah increased at the time of the Rightly Guided Caliph `Uthman ibn`Affan (may Allah be pleased with him), so he decided to add the another adhan. It is called the first adhan, and is given to alert the people to the fact that this day is Friday, so that they may prepare themselves and hasten to the prayer before the regular adhan that is given after the sun has passed the meridian.
The Companions followed him in that at his time, such as `Ali (may Allah be pleased with him), `Abd ar-Rahmaan ibn `Awf, Az-Zubayr ibn al-`Awwam, Talhah ibn `Ubaydullah, and other senior Companions.
This is how the Muslims started to do it in most regions, following the action of the Rightly Guided Caliph, `Uthman ibn `Affan; he was followed in that by the Rightly Guided Caliph ‘Ali (may Allah be pleased with him) and the rest of the Companions.
The point is that this happened during the caliphate of `Uthman and subsequently, and it has remained the practice of most of the Muslims throughout the regions and throughout the centuries until the present day, following the precedent that was set by `Uthman (may Allah be pleased with him), on the basis of his ijtihad and sincerity towards the Muslims.
And there is nothing wrong with that, because the Messenger said: “I urge you to adhere to my Sunnah and the way of the Rightly Guided Caliphs; hold fast to it.” He (`Uthman) was one of the Rightly Guided Caliphs, and the interest served by that is quite clear. Hence this was adopted by Ahl as-Sunnah wal-Jama`ah, and they did not see anything wrong with it, because it was part of the practice of the Rightly Guided Caliphs `Uthman and `Ali and those of the Companions who were present at that time (may Allah be pleased with them all).
Almighty Allah knows best.