Musnad Ahmad 30:1811

Qutaybah narrated to us, Layth ibn Sa'd narrated to us, from Ibn 'Ajlan, from al-Qa'qa' ibn Hakim, from Abu Salih, from Abu Hurayrah, from the Messenger of Allah (may Allah bless him and grant him peace), that he said: "Upon every soul of the children of Adam, its share of adultery has been written; it will inevitably reach it. The adultery of the eyes is the looking, the adultery of the ears is the listening, the adultery of the hand is the grasping, the adultery of the foot is the walking, and the adultery of the tongue is the speech. The heart desires and wishes, and the private part confirms that or denies it."