Bilal was ordered to pronounce the call to prayer in pairs and the iqamah singly.
حدثنا قتيبة حدثنا عبد الوهاب الثقفي ويزيد بن زريع عن خالد الحذاء عن أبي قلابة
أمر بلال أن يشفع الأذان ويوتر الإقامة
Agreed upon [by Bukhari and Muslim]. Bukhari transmitted it in the Book of Adhan, Chapter on the Beginning of the Adhan, no. 603, and Muslim in the Book of Prayer, Chapter on the Command to Pair the Adhan and Make the Iqamah Single Except for the Phrase of the Iqamah, Which Is Doubled, no. 378, from the hadith of Khalid Haddan through this route. On this subject there is also a report from Ibn Umar [Abu Dawud, no. 510, 511].