Musnad Ahmad 482:85

Waki' narrated to us, 'Abd al-Hamid ibn Ja'far al-Ansari narrated to us, from Yazid ibn Abi Habib, from Marthad ibn 'Abd Allah al-Yazani, from 'Uqbah ibn 'Amir al-Juhani, 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."