Musnad Ahmad 482:71

'Abdullah ibn Yazid narrated to us, Ibn Lahi'ah and Hashim narrated to us, Layth narrated to us, from Yazid ibn Abi Habib, from Abu al-Khayr Marthad ibn 'Abdullah al-Yazani, from 'Uqbah ibn 'Amir al-Juhani, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Indeed, the most deserving of conditions for you to fulfill are those by which you make the private parts lawful."