أُمِرَ بِلاَلٌ أَنْ يَشْفَعَ، الأَذَانَ وَأَنْ يُوتِرَ الإِقَامَةَ إِلاَّ الإِقَامَةَ.
Anas ibn Malik reported: Bilal was commanded to say the adhan in pairs and the iqamah singly, except (the words of) the iqamah.
also in Sahih al-Bukhari 603, Sahih al-Bukhari 606, Sahih al-Bukhari 607