Musnad Ahmad 2:131

Ishaq ibn Isa narrated to us, Ibn Lahi'ah narrated to us, from Ja'far ibn Rabi'ah, from al-Zuhri, from Muharrir ibn Abi Hurayrah, from his father, from Umar ibn al-Khattab: That the Prophet, peace and blessings of Allah be upon him, forbade *al-'azl* (coitus interruptus) with a free woman except with her permission.