The Messenger of Allah ﷺ commanded Bilal to pronounce the call to prayer in pairs and the iqamah singly.
أخبرنا قتيبة بن سعيد قال حدثنا عبد الوهاب عن أيوب عن أبي قلابة
إن رسول الله ﷺ أمر بلالا أن يشفع الأذان وأن يوتر الإقامة
Reported by Muslim, Prayer, chapter on the command to pair the adhan and to make the iqamah singular, from the hadith of Abdul-Wahhab, and by Bukhari, Adhan, chapter on saying the adhan two by two, no. 605, from the hadith of Ayub Sakhtiyani, with the same wording. It is also in al-Kubra, no. 1592.