Musnad Ahmad 482:12

Yahya ibn Sa'id narrated to us, from 'Abd al-Hamid ibn Ja'far, who said: Yazid ibn Abi Habib narrated to me, from Marthad ibn 'Abd Allah al-Yazani, 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."