"The zina of the eyes is the gaze (at that which is unlawful, eg. Nudity); the zina of the ears is to listen (to talks of nudity which excites the carnal desire); the zina of the tongue is to speak (what is evil); the zina of the hand is to touch (the female which is unlawful to you); the zina of the feet is to walk (towards immorality); the zina of the heart is to desire (what is unlawful), and it is the private parts which either commits or shuns the actual act of fornication."

Muslim 4/1397, no. 6421, 6422