Sahih al-Bukhari Book 10, Hadith 4
When the number of Muslims increased they discussed the question as to how to know the time for
the prayer by some familiar means. Some suggested that a fire be lit (at the time of the prayer) and
others put forward the proposal to ring the bell. Bilal was ordered to pronounce the wording of Adhan
twice and of the Iqama once only.
Bilal was ordered to repeat the wording of the Adhan for prayers twice, and to pronounce the wording of the Iqama once except "Qad-qamat-is-salat".
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).