Musnad Abu Ya'la al-Mawsili 152:296

Abu Khaythama narrated to us, Affan narrated to us, Hammam ibn Yahya narrated to us, Asim narrated to us, from Abu al-Duha, from Masruq, from Abdullah, from the Prophet, may Allah bless him and grant him peace, who said: The two eyes commit adultery, and the two hands commit adultery, and the two feet commit adultery, and the private part commits adultery.