Musnad Abu Ya'la al-Mawsili 160:42

Abu Khaythama narrated to us, Mu'adh ibn Hisham narrated to us, my father narrated to me, from Qatada, from Khallas, from Abu Rafi', from Abu Hurayra, that the Prophet, peace and blessings of Allah be upon him, said: "The believer has two wives; he sees the marrow of their shins from beneath their garments."