“A deed is not accepted unless it is sincere and correct: sincere, done for Allah alone, and correct, in accordance with the Sunnah.”

Al-Fudayl ibn ‘Iyad (d. 187 AH / 803 CE) said this explaining the Qur’anic phrase “best in deed” in Surat al-Mulk: the best deed, he said, is “the most sincere and the most correct.” Abu Nu‘aym records it in Hilyat al-Awliya’, and Ibn al-Qayyim quotes it twice in Madarij as-Salikin, with the definitions that follow it: sincere is what is done for Allah, and correct is what follows the Sunnah.