Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

We came to the Prophet and stayed with him for twenty days and nights. We were all young and of about the same age. The Prophet was very kind and merciful. When he realized our longing for our families, he asked about our homes and the people there and we told him. Then he asked us to go back to our families and stay with them and teach them (the religion) and to order them to do good things. He also mentioned some other things which I have (remembered or) forgotten. The Prophet then added, "Pray as you have seen me praying and when it is the time for the prayer one of you should pronounce the Adhan and the oldest of you should lead the prayer.