Musnad Ahmad 30:1477

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abd al-Rahman al-A'raj narrated to us, from Abu Hurairah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Every son of Adam has been afflicted with a share of adultery, inevitably. The adultery of the eye is the looking, and the adultery of the hand is the touching. The soul desires and craves, and the private part confirms that or denies it."