The word(s) "sincere" appears 18 time(s) in 18 verse(s) in Quran in Mohsin Khan translation.

(11) Say (O Muhammad صلى الله عليه وسلم): "Verily I am commanded to worship Allah (Alone) by obeying Him and doing religious deeds sincerely for His sake only.

(12) Say (O Muhammad صلى الله عليه وسلم): "Allah Alone I worship by doing religious deeds sincerely for His sake only (and not to show off, and not to set up rivals with Him in worship.)"

(13) So, call you (O Muhammad صلى الله عليه وسلم and the believers) upon (or invoke) Allah making (your) worship pure for Him (Alone) (by worshipping none but Him and by doing religious deeds sincerely for Allah's sake only and not to show off and not to set up rivals with Him in worship), however much the disbelievers (in the Oneness of Allah) may hate (it).

(14) He is the Ever Living, La ilaha illa Huwa (none has the right to be worshipped but He); so invoke Him making your worship pure for Him Alone (by worshipping Him Alone, and none else, and by doing righteous deeds sincerely for Allah's sake only, and not to show off, and not setting up rivals with Him in worship). All the praises and thanks be to Allah, the Lord of the 'Alamin (mankind, jinn and all that exists).

(15) Those who deny the Book (this Qur'an), and that with which We sent Our Messengers (i.e. to worship none but Allah Alone sincerely, and to reject all false deities and to confess resurrection after the death for recompense) they will come to know (when they will be cast into the Fire of Hell).

(16) (It will be said): "This is what you were promised - (it is) for those oft-returning (to Allah) in sincere repentance, and those who preserve their covenant with Allah (by obeying Him in all what He has ordered, and worshipping none but Allah Alone, i.e. follow Allah's religion - Islamic Monotheism).

(17) It may be if he divorced you (all) that his Lord will give him instead of you, wives better than you - Muslims (who submit to Allah), believers, obedient (to Allah), turning to Allah in repentance, worshipping Allah sincerely, given to fasting or emigrants (for Allah's sake), previously married and virgins.

(18) O you who believe! Turn to Allah with sincere repentance! It may be that your Lord will expiate from you your sins, and admit you into Gardens under which rivers flow (Paradise) - the Day that Allah will not disgrace the Prophet (Muhammad صلى الله عليه وسلم) and those who believe with him. Their Light will run forward before them and (with their Records - Books of deeds) in their right hands. They will say: "Our Lord! Keep perfect our Light for us [and do not put it off till we cross over the Sirat (a slippery bridge over the Hell) safely] and grant us forgiveness. Verily, You are Able to do all things."