(6) باب إِفْرَادِ الإِقَامَةِ
It was narrated that Anas bin Malik said:
"They looked for something by means of which they could call out informing of (the time of) the prayer. Then Bilal was commanded to say the phrases of the Adhan twice and the phrases of the Iqamah once."
Reference: Sunan Ibn Majah 729
In-book reference: Book 3, Hadith 24
English translation: Vol. 1, Book 3, Hadith 729