The religion of Allaah is the most sincere of it and the most correct of it

Al Fudayl bin Iyaad (rahimahullaah) said,”The religion of Allaah is the most sincere of it and the most correct of it.” It was said, “O Abu Ali, what is the most sincere of it and the most correct of it?” He said,

If an action is sincere and it is not correct then it will not be accepted, and if it is correct and it is not sincere then it will not be accpted, until it is sincere and correct, and it will be sincere if it is (done) for Allaah, and it will be correct if it is done upon the Sunnah.

Narrated by Ibn Abi Dunya in his book “Al-Ikhlaas wan Niyyah” p. 50-51, and
by Abi Na’eem in “Al Hilyah” v.8 p.95, 
Select Issues Concerning Du’aa` (Supplication) Part One