Sunan Abu Dawood, Chapter: Prayer (Kitab Al-Salat)

The words of adhan were pronounced from the time of the Apostle of Allah (peace_be_upon_him) twice in pairs (i.e. four times) each, and the words of iqamah were pronounced once in pairs (twice each), except that the phrase "The time for prayer has come" would be pronounced twice. When we heard iqamah, we would perform ablution, and go out for prayer. Shu'bah said: I did not hear AbuJa'far narrating any other tradition except this one.