Sahih Bukhari

Narrated Abu Qilaba:Anas said, ""Bilal was ordered to pronounce the wording of Adhan twice and of Iqama once only."" The sub narrator Isma`il said, ""I mentioned that to Aiyub and he added (to that), ""Except Iqama (i.e. Qad-qamat-is-salat which should be said twice)