Fudayl ibn Iyaad (rahimahullah) said:
The good action is the most sincere and the most correct.
He was asked: “Oh Abaa Alee, what is the most sincere and the most correct?
Indeed the action which was sincerely for Allah but not correct, is not accepted, and if it was correct but not sincere it is not accepted, until it is sincere and correct.
And Sincere (means): that which was for Allah (alone).
And Correct (means): that which is (done) according to the Sunnah.
Madaarij As-Saalikeen Vol.1 Pg. 82