"Written upon the son of Adam is his share of zinā, which he will inevitably reach: the zinā of the eyes is the gaze, the zinā of the ears is listening, and the zinā of the tongue is speech."
The narration continues, in substance: the hand and the foot have their share also, the heart desires and wishes, and the private parts either confirm that or deny it.
Narrated from Abū Hurayrah. Ṣaḥīḥ Muslim 2658a (Kitāb al-Qadar); also Ṣaḥīḥ al-Bukhārī (muttafaq ʿalayh).
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In The Gaze and the Feed
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The Prophet ﷺ reached for the heaviest word available to him. He did not call the gaze a step toward zinā, nor a cause of it. He called it the zinā of the eyes — giving the sin's own name to its smallest form.
That naming does deliberate work. It refuses the mental compartment nearly all of us maintain, in which the serious thing stands over here and a broad, harmless country of looking and watching and consuming stretches out over there. The hadith collapses that country into one continuous act distributed among the limbs, and the eyes are where it begins.
Qur'anic Arabic is reproduced in the Uthmani script. Hadith Arabic is reproduced from the published collections named above. The English is a rendering informed by widely used translations, offered as an aid to meaning and not a substitute for the Arabic. Verify what is cited, and consult people of knowledge on anything affecting practice.