Q 10: What does "Sincerity of intention" mean?
A ---It means that all the words and deeds of Allah's slave made manifest and concealed, must be for nothing save to seek the Countenance of Allah, the Most High.
Allah the Almighty says: (And they were commanded not, but that they should worship Allah, and worship none but Him Alone]}. (Al-Bayyinah, 5)
He says: {And who has (in mind) no favor from anyone to be paid back. Except to seek the Countenance of his Lord, the Most High}.
He says: {We feed you seeking Allah's Countenance only. We wish for no reward, nor thanks from you}. (Al-Insan, 9)
He says: {Whosoever desires (by his deeds) the reward of the Hereafter, We give him increase in his reward, and whosoever desires the reward of this world (by his deeds), We give him thereof (what is decreed for him), and he has no portion in the Hereafter}. (Ash-Shura, 20)
Reference: 200 FAQ On Muslims Belief - Shiekh Hafiz Ibn Ahmed Al Hakami