صحیح ابن حبان
Ibn Hibban
كتاب الصلاة
Book of Prayer
باب الأذان - ذكر الخبر الدال على أن النبي صلى الله عليه وسلم هو الآمر لبلال تثنية الأذان وإفراد الإقامة لا غيره
Chapter on Adhan - Report indicating the Prophet himself ordered Bilal to double the adhan and single the iqamah.
1 hadith
Hadith 1678
Sayyiduna Anas (may Allah be pleased with him) narrates: The people searched for something through which they could inform others, and that thing would be a specific sign for prayer. Sayyiduna Bilal (may Allah be pleased with him) was commanded to say the words of the adhan in even numbers and the words of the iqamah in odd numbers.