The person who has a sincere intention is the one who has a sound heart; because a sound heart, free from thoughts about forbidden things, comes from making your intention purely for Allah in all matters.
يَوْمَ لَا يَنفَعُ مَالٌ وَلَا بَنُونَ. إِلَّا مَنْ أَتَى اللَّهَ بِقَلْبٍ سَلِيمٍ
The Holy Prophet said, 'The intention of the believer is better than his action,' and also, 'Actions are by intentions, and every man will obtain what he had intended.' The slave of Allah must therefore have sincere intention in every moment of action and stillness because then he will not be heedless. Those who are heedless have been censured by Allah:
إِنْ هُمْ إِلَّا كَالْأَنْعَامِ بَلْ هُمْ أَضَلُّ سَبِيلًا
أُوْلَـئِكَ هُمُ الْغَافِلُونَ
Intention appears from the heart, according to the purity of knowledge. It varies as belief varies, at different moments in its strength and weakness. The selfishness and passion of those with sincere intention is subjugated to the power of the glorification of Allah and modesty before Him. He is by his nature, his appetites and his own desires, in a state of discomfort, and yet people find ease at his hand.