(2) باب الأَذَانُ مَثْنَى مَثْنَى
Narrated Anas:
Bilal was ordered to repeat the wording of the Adhan for prayers twice, and to pronounce the wording of the Iqama once except "Qad-qamat-is-salat".
Reference: Sahih al-Bukhari 605
In-book reference: Book 10, Hadith 3
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 579