The word(s) "sincere" appears 38 time(s) in 38 verse(s) in Quran in Abdullah Yusuf Ali translation.

(31) He is the Living (One): There is no god but He: Call upon Him, giving Him sincere devotion. Praise be to Allah, Lord of the Worlds!

(32) Only those are Believers who have believed in Allah and His Messenger, and have never since doubted, but have striven with their belongings and their persons in the Cause of Allah: Such are the sincere ones.

(33) They impress on thee as a favour that they have embraced Islam. Say, "Count not your Islam as a favour upon me: Nay, Allah has conferred a favour upon you that He has guided you to the faith, if ye be true and sincere.

(34) (A voice will say:) "This is what was promised for you,- for every one who turned (to Allah) in sincere repentance, who kept (His Law),

(35) And those who believe in Allah and His messengers- they are the sincere (lovers of Truth), and the witnesses (who testify), in the eyes of their Lord: They shall have their Reward and their Light. But those who reject Allah and deny Our Signs,- they are the Companions of Hell-Fire.

(36) (Some part is due) to the indigent Muhajirs, those who were expelled from their homes and their property, while seeking Grace from Allah and (His) Good Pleasure, and aiding Allah and His Messenger: such are indeed the sincere ones:-

(37) O ye who believe! Turn to Allah with sincere repentance: In the hope that your Lord will remove from you your ills and admit you to Gardens beneath which Rivers flow,- the Day that Allah will not permit to be humiliated the Prophet and those who believe with him. Their Light will run forward before them and by their right hands, while they say, "Our Lord! Perfect our Light for us, and grant us Forgiveness: for Thou hast power over all things."

(38) And they have been commanded no more than this: To worship Allah, offering Him sincere devotion, being true (in faith); to establish regular prayer; and to practise regular charity; and that is the Religion Right and Straight.