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).
Hadith Referenceصحيح البخاري / كتاب الجمعة / 915
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة