Sahih Bukhari Volume 1, Book 10, Hadith 820

قَالَ فَأَتَيْنَا النَّبِيَّ صلى الله عليه وسلم فَأَقَمْنَا عِنْدَهُ فَقَالَ ‏"‏ لَوْ رَجَعْتُمْ إِلَى أَهْلِيكُمْ صَلُّوا صَلاَةَ كَذَا فِي حِينِ كَذَا، صَلُّوا صَلاَةَ كَذَا فِي حِينِ كَذَا، فَإِذَا حَضَرَتِ الصَّلاَةُ فَلْيُؤَذِّنْ أَحَدُكُمْ وَلْيَؤُمَّكُمْ أَكْبَرُكُمْ ‏"‏

Malik bin Huwairith said, "We came to the Prophet (after embracing Islam) and stayed with him. He said to us, 'When you go back to your families, pray such and such a prayer at such and such a time, pray such and such a prayer at such and such a time, and when there is the time for the prayer then only of you should pronounce the Adhan for the prayer and the oldest of you should lead the prayer."