´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."
Hadith ReferenceSunan Ibn Majah / The Book of the Adhan and the Sunnah Regarding It / Hadith: 729
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم