"The Messenger of Allah (S.A.W) commanded Bilal to say the phrases of the Adhan twice and the phrases of the Iqamah once."
Sunan an-Nasa'i Book 7, Hadith 2
← Previous Hadith "When the Muslims arrived in Al-Madinah they used to gather and try to figure out the time for prayer, and no one gave the call to prayer. One day they spoke about that; some of them said: 'Let us use a bell like the Christians do;' others said, 'No, a horn like the Jews have.' 'Umar, may ,Allah be pleased with him, said: 'Why don't you send a man to announce the time of prayer?' The Messenger of Allah (S.A.W) said: 'O Bilal, get up and give the call to prayer.'"

Sunan an-Nasa'i Book 7, Hadith 1 Next Hadith → "At the time of the Messenger of Allah (S.A.W) the phrases of the Adhan were said twice and the phrases of the Iqamah were said once, except that you should say: 'Qad Qamatis-Salah, Qad Qamatis-Salah (prayer is about to begin, prayer is about to begin).'"

Sunan an-Nasa'i Book 7, Hadith 3