Musnad Abu Ya'la al-Mawsili 107:21

Abu Khaythama narrated to us, Waki’ ibn al-Jarrah narrated to us, ‘Abd al-Hamid ibn Ja’far al-Ansari narrated to me, from Yazid ibn Abi Habib, 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 worthy of conditions to be fulfilled are those by which you make the private parts lawful."