(24) باب الْجُلُوسِ عَلَى الْمِنْبَرِ عِنْدَ التَّأْذِينِ
Narrated As-Sa'ib bin Yazid I:
`Uthman bin `Affan introduced the second Adhan on Fridays when the number of the people in the mosque increased. Previously the Adhan on Fridays used to be pronounced only after the Imam had taken his seat (on the pulpit).
Reference: Sahih al-Bukhari 915
In-book reference: Book 11, Hadith 39
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 13, Hadith 38