Musannaf Ibn Abi Shayba 9:548

Waki' narrated to us, from 'Abd al-Hamid ibn Ja'far, from Yazid ibn Abi Habib, from Abu al-Khayr, from 'Uqbah ibn 'Amir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the most deserving of conditions to be fulfilled are those by which you make the private parts lawful."