Sahih Al Bukhari Hadith Number 915

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).

11  Friday Prayer
Hadith number 915
Volume no 2
Chapter: 24 (To sit on the pulpit while Adhan is pronounced)