(3) باب الإِقَامَةُ وَاحِدَةٌ، إِلاَّ قَوْلَهُ قَدْ قَامَتِ الصَّلاَةُ
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. Qadqamat- is-salat which should be said twice).
Reference: Sahih al-Bukhari 607
In-book reference: Book 10, Hadith 5
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 581