Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

We went to the Prophet and we were all young men and stayed with him for about twenty nights. The Prophet was very merciful. He said, "When you return home, impart religious teachings to your families and tell them to offer perfectly such and such a prayer at such and such a time and such and such a prayer at such and such a time. And al the time of the prayer one of you should pronounce the Adhan and the oldest of you should lead the prayer."