Sahih Bukhari | Volume 1 | Hadith Number 38
Narrated by Abu Huraira
The Prophet said, "Religion is very easy and whoever overburdens himself in his religion will not be able to continue in that way. So you should not be extremists, but try to be near to perfection and receive the good tidings that you will be rewarded; and gain strength by worshipping in the mornings, the nights." (See Fath-ul-Bari, Page 102, Vol 1).