´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 Referenceسنن نسائي / كتاب الأذان / 628
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:إسناده صحيح، متفق عليه
´It was narrated that Ibn 'Umar said:` "At the time of the Messenger of Allah (S.A.W) the phrases of the Adhan were said twice and the phrases of the Iqamah were said once, except that you should say: 'Qad Qamatis-Salah, Qad Qamatis-Salah (prayer is about to begin, prayer is about to begin).'"