Sunan An-Nasai, Chapter: The Book of the Adhan (The Call to Prayer)

the Muadhdhin of the Jami Masjid, said: "I asked Ibn Umar about the Adhan and he said: 'At the time of the Messenger of Allah (saw), the phrases of the Adhan were recited twice and the phrases of the Iqamah once, except that you should say (the phrase) Qad qamat is-salah (prayer is about to begin) twice. When we heard 'prayer is about to begin' we would perform Wudu and go out to pray.'" (Sahih)