Bilal was commanded to pronounce the call to prayer in pairs and the iqamah singly, except for the words of the iqamah itself.
أمر بلال أن يشفع الأذان، وأن يوتر الإقامة إلا الإقامة.
Bukhari related it in the Book of Adhan (605) and Muslim in the Book of Prayer (378), both of them through Ayub, from Abu Qilaba, from Anas, who narrated it. Some of the routes of this hadith have already been mentioned in the chapter on the beginning of the adhan.