Bilal was ordered to pronounce the call to prayer in pairs and the iqamah singly.
حدثنا عبد الوهاب حدثنا أيوب عن أبي قلابة
أمر بلال أن يشفع الأذان ويوتر الإقامة
Its isnad is sahih. Abu Qilaba is Abdullah bin Zayd Jarmi.The hadith was narrated by Bukhari 2/82 no. 606, Fath al-Bari, in the chapter on the adhan, and Muslim 1/286 no. 378, in the chapter on prayer, on the command to make the adhan in pairs, and Abu Dawud 1/141 no. 508, and Tirmidhi 1/370 no. 193, who said it is hasan sahih, and Nasai 2/3 no. 627, and Ibn Majah 1/241 no. 730.