Silsilah as-Sahihah 91 — Definition of sin

Sayyiduna Abu Umamah (may Allah be pleased with him) narrates that a man asked: "O Messenger of Allah! What is faith?" He (peace and blessings be upon him) replied: "When your good deed pleases you and your bad deed displeases you, then you are a believer." He said: "O Messenger of Allah! What is sin?" He (peace and blessings be upon him) replied: "When something pricks your heart, then leave it (considering it a sin)."