´It was narrated that Anas said:` "The Messenger of Allah (S.A.W) commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once."
Hadith ReferenceSunan an-Nasa'i / The Book of the Adhan (The Call to Prayer) / Hadith: 628
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:إسناده صحيح، متفق عليه