`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 : Book 11, Hadith 39 USC-MSA web (English) : Vol. 2, Book 13, Hadith 38 (deprecated)
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