(2) باب الأَمْرِ بِشَفْعِ الأَذَانِ وَإِيتَارِ الإِقَامَةِ
Anas reported:
Bilal was commanded (by the Holy Prophet) to repeat the phrases twice in Adhan, and once in lqama. (ID)
Reference (ID): Sahih Muslim 378 d (ID)
In-book reference (ID): Book 4, Hadith 5 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 739 (ID)