Hadith: The son of Adam has been destined his share of fornication, which he will inevitably acquire. The eyes fornicate by looking, the ears fornicate by listening, the tongue fornicates by speaking, the hand fornicates by hitting, the foot fornicates by stepping. The heart loves and wishes. The genitals prove or disapprove that

عن أبي هريرة رضي الله عنه أن النبي صلى الله عليه وسلم قال: «كُتب على ابن آدم نَصِيبُه من الزِنا مُدْرِكُ ذلك لا مَحَالة: العينان زِناهما النَظر، والأُذنان زِناهما الاستماع، واللسان زِناه الكلام، واليَدُ زِناها البَطْش، والرِّجل زِناها الخُطَا، والقلب يَهْوَى ويتمنى، ويُصَدِّق ذلك الفَرْج أو يُكذِّبُه».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: ''The son of Adam has been destined his share of fornication, which he will inevitably acquire. The eyes fornicate by looking, the ears fornicate by listening, the tongue fornicates by speaking, the hand fornicates by hitting, the foot fornicates by stepping. The heart loves and wishes. The genitals prove or disapprove that.’'
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Except for those whom Allah has protected, everyone ends up fornicating. The Prophet (may Allah's peace and blessings be upon him) mentioned examples of fornication. The eyes fornicate, i.e. when a man looks at a woman, even if not lustfully, who is not from his Mahrams, this is one kind of fornication. The ears fornicate by listening to and enjoying the voice of a woman. The hands fornicate by touching. The feet fornicate by going to a place of immorality, or following a woman that one unlawfully desires. The heart fornicates by being inclined to women. The genitals prove that when they commit real fornication. If they do not fornicate, then this will be disapproval of other organs’ fornication. From this, men should learn to be cautious not to get attached to women, listen to their voice, see them, touch them, pursue them, nor think of their seduction. These are types of fornication which every chaste and sensible man would resist.