Musnad Ahmad 30:3789

'Abd al-Samad ibn 'Abd al-Warith narrated to us, Hammad narrated to us, from Suhayl, 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 his share of adultery. The adultery of the eyes is the looking, the adultery of the hands is the grasping, the adultery of the feet is the walking, the adultery of the mouth is the kissing, and the heart desires and wishes, and the private part confirms that or denies it." And he formed a circle with his ten [fingers], then inserted his index finger into it. Abu Hurayrah bore witness to that with his flesh and his blood.