An-Nawwās ibn Sam’ān (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: “Righteousness is good morals and sinfulness is what your heart is not at ease with and you hate that people know about it.”
Wābisah ibn Ma‘bad (may Allah be pleased with him) reported: I came to the Messenger of Allah (may Allah's peace and blessings be upon him) and he said: "Have you come asking about righteousness?" I replied in the affirmative, so he said: "Consult your heart, for righteousness is what your soul and heart feel at ease with, whereas sin is what your soul and heart feel uneasy with even if people keep on giving you their opinions in their favor."
The Hadīth here explained righteousness as good morals manifested in the relationship between one and his Lord, one and his Muslim brother and one and people in general, Muslims and non-Muslims.
Righteousness, as mentioned in the second Hadīth, could be defined as whatever one’s soul feels at ease with and it usually feels at ease with good deeds and sayings.
On the other hand, sinfulness is whatever one’s soul feels uneasy with. It is like recurring doubt in the heart that one should avoid out of piety and for fear of committing what is prohibited. Piety is leaving them all and going only for what one’s heart feels relaxed with.
Whatever one’s heart feels uneasy with is sinful, even if people say the otherwise. That is only in case one’s heart is full of faith and the one giving him Fatwa has no Shariah-based evidence. However, if there is a Shariah-based evidence, then the one seeking Fatwa will have to abide by the Fatwa given even if he does not feel at ease with it.