Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 736
Anas reported: Bilal was commanded (by the Apostle of Allah) to repeat (the phrases of) Adhan twice and once in Iqama. The narrator said: I made a men- tion of it before Ayyub who said: Except for saying: Qamat-is-Salat [the time for prayer has come].