Bilal's call to prayer was made in pairs, and his iqamah was made singly.
حدثنا هشام بن عمار حدثنا عبد الرحمن بن سعد
أن أذان بلال كان مثنى مثنى وإقامته مفردة
No references are available.
[Its chain of transmission is daif] See hadith no. 710 for its defect, and the preceding hadith (729) suffices in its place.