Al-Fudayl b. ʿAyyāḍ [187H] – Allah have mercy on him – said:
“Allah ‘azza wa jalla accepts only those deeds which are both correct and sincere (pure). If the deed is done correctly but not sincerely, it will not be accepted. And if it is sincere but not correct, it will not be accepted.” He was asked, “Abū ‘Alī! What is the sincere and correct deed?” He replied, “The sincere deed is one that is done only for Allah ‘azza wa jall. And the correct deed is one done according to the Sunnah.”
Abū Nu’aym, Hilyah Al-Awliyā` Vol.8 p95.