An-Nawwas bin Sam'an (RAA) narrated, 'I asked the Messenger of Allah (ﷺ) about virtue and sin and he replied, "The essence of virtue is (manifested in) good morals (Akhlaq) whereas sinful conduct is that which turns in your heart (making you feel uncomfortable) and you dislike that it would be disclosed to other people." Related by Muslim.
English reference: Book 16, Hadith 1483
Arabic reference: Book 16, Hadith 1439