Bilal was commanded to make the call to prayer in pairs and the iqamah singly. Hammad added in his version, "except for the iqamah."
حدثنا سليمان بن حرب وعبد الرحمن بن المبارك قالا حدثنا حماد عن سماك بن عطية ح و حدثنا موسى بن إسمعيل حدثنا وهيب جميعا عن أيوب عن أبي قلابة
أمر بلال أن يشفع الأذان ويوتر الإقامة زاد حماد في حديثه إلا الإقامة
Bukhari narrated it in the Book of Adhan, Chapter on the adhan being said in pairs, hadith no. 605, from Sulayman bin Harb, and Muslim narrated it in the Book of Prayer, Chapter on the command to say the adhan in pairs and the iqamah singly, except for the phrase of the iqamah which is said in pairs, hadith no. 378, both from the hadith of Ayub Sakhtiyani through this same chain.