Musnad Ahmad 30:1406

'Affan narrated to us, Hammad ibn Salamah narrated to us, Suhayl ibn Abi Salih informed us, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Every son of Adam has a share of adultery. The two eyes commit adultery, and their adultery is looking. The two hands commit adultery, and their adultery is grasping. The two feet commit adultery, and their adultery is walking. The mouth commits adultery, and its adultery is kissing. The heart desires and wishes, and the private part confirms that or denies it."