Sahih Muslim Book 4, Hadith Number 1353.

Chapter : Desirability of observing the morning prayer at earlier hour and that is the time when there is darkness before dawn and the exposition about the length of recitation in it.

Abu Dharr reported: The Messenger of Allah (may peace be upon him) said to me: How would you act when you are under the rulers who would delay the prayer beyond its prescribed time, or they would make prayer a dead thing as far as its proper time is concerned? I said: What do you command? He (the Holy Prophet) said: Observe the prayer at Its proper time, and if you can say it along with them do so, for it would be a supererogatory prayer for you. Khalaf (one of the narrators in the above Hadith) has not mentioned “beyond their (prescribed) time”.