Sahih al-Bukhari 607
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).
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا خَالِدٌ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسٍ، قَالَ أُمِرَ بِلاَلٌ أَنْ يَشْفَعَ، الأَذَانَ، وَأَنْ يُوتِرَ الإِقَامَةَ‏.‏ قَالَ إِسْمَاعِيلُ فَذَكَرْتُ لأَيُّوبَ فَقَالَ إِلاَّ الإِقَامَةَ‏.‏