Allah said,

[إِذَا نُودِىَ لِلصَّلَوةِ مِن يَوْمِ الْجُمُعَةِ]

(When the call is proclaimed for the Salah on Friday,) referring to the Adhan which was called, during the time of the Prophet , when he came out of his house and sat on the Minbar. The Adhan would be called before the Prophet near the door of the Masjid. As for the earlier Adhan that the Leader of the faithful, `Uthman bin Affan added, it was done because the Muslims increased in number during his time. Al-Bukhari recorded that As-Sa'ib bin Yazid said, "In the lifetime of the Prophet , Abu Bakr and `Umar, the Adhan for the Friday prayer was pronounced while the Imam sat on the pulpit. But during `Uthman's later time when the Muslims increased in number, an additional call was pronouced upon Az-Zawra', meaning the Adhan was called upon the house which was called Az-Zawra''' Az-Zawra' was the tallest house in Al-Madinah near the Masjid.