The word(s) "sincere" appears 18 time(s) in 18 verse(s) in Quran in Muhammad Habib Shakir translation.

(11) Surely We have revealed to you the Book with the truth, therefore serve Allah, being sincere to Him in obedience.

(12) Now, surely, sincere obedience is due to Allah (alone) and (as for) those who take guardians besides Him, (saying), We do not serve them save that they may make us nearer to Allah, surely Allah will judge between them in that in which they differ; surely Allah does not guide him aright who is a liar, ungrateful.

(13) Say: I am commanded that I should serve Allah, being sincere to Him in obedience.

(14) Say: Allah (it is Whom) I serve, being sincere to Him in my obedience:

(15) Therefore call upon Allah, being sincere to Him in obedience, though the unbelievers are averse:

(16) He is the Living, there is no god but He, therefore call on Him, being sincere to Him in obedience; (all) praise is due to Allah, the Lord of the worlds.

(17) O you who believe! turn to Allah a sincere turning; maybe your Lord will remove from you your evil and cause you to enter gardens beneath which rivers flow, on the day on which Allah will not abase the Prophet and those who believe with him; their light shall run on before them and on their right hands; they shall say: Our Lord! make perfect for us our light, and grant us protection, surely Thou hast power over all things.

(18) And they were not enjoined anything except that they should serve Allah, being sincere to Him in obedience, upright, and keep up prayer and pay the poor-rate, and that is the right religion.