Musnad Ahmad 30:1237

Abu al-Nadr narrated to us, al-Mubarak narrated to us, from al-Hasan, from Abu Hurayrah—and I do not know it except from the Prophet, may Allah bless him and grant him peace—who said: "The eye commits adultery, and the heart commits adultery. The adultery of the eye is the looking, and the adultery of the heart is the wishing. And the private part confirms all of that or denies it."