The Messenger of Allah ﷺ said, "Every son of Adam has had his share of fornication decreed for him, and he will inevitably meet it. As for the eye, its fornication is the gaze, and what appears before it bears witness to that. The tongue's fornication is speech, and the heart's is longing. Then the private parts either confirm all of this or give it the lie."
وقال رسول الله ﷺ كتب على ابن آدم نصيبه من الزنا أدرك لا محالة فالعين زنيتها النظر ويصدقها الأعراض واللسان زنيته النطق والقلب التمني والفرج يصدق ما ثم ويكذب
A sahih hadith, narrated by Bukhari through the route of Mamar, from Ibn Tawus, from his father, from Ibn Abbas, from Abu Hurairah, 8:54. Muslim also narrated it, 8:52, through the same route. Abu Dawud narrated it in the chapter on marriage, from Musa bin Ibrahim, and Tirmidhi narrated it in the chapter on purification, with the wording, "Every son of Adam has his share of fornication."