Sahih Bukhari | Volume 1 | Hadith Number 535

Narrated by Jabir bin 'Abdullah
The Prophet used to pray the Zuhr at mid-day, and the 'Asr at a time when the sun was still bright, the Maghrib after sunset (at its stated time) and the Isha at a variable time. Whenever he saw the people assembled (for 'Isha prayer) he would pray earlier and if the people delayed, he would delay the prayer. And they or the Prophet used to offer the Fajr Prayers when it still dark.