Sahih Bukhari Volume 001, Book 011, Hadith Number 581.

Narated By Abu Qilaba : Anas said, "Bilal was ordered to pronounce the wording of Adhan twice and of Iqama once only." The sub narrator Isma'li said, "I mentioned that to Aiyub and he added (to that), "Except Iqama (i.e. Qad-Qamatis-Salat which should be said twice)."