It was narrated that Anas bin Malik said:

"They looked for something by means of which they could call out informing of (the time of) the prayer. Then Bilal was commanded to say the phrases of the Adhan twice and the phrases of the Iqamah once."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْجَرَّاحِ، حَدَّثَنَا الْمُعْتَمِرُ بْنُ سُلَيْمَانَ، عَنْ خَالِدٍ الْحَذَّاءِ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ الْتَمَسُوا شَيْئًا يُؤْذِنُونَ بِهِ عِلْمًا لِلصَّلاَةِ فَأُمِرَ بِلاَلٌ أَنْ يَشْفَعَ الأَذَانَ وَيُوتِرَ الإِقَامَةَ ‏.‏