Musannaf Ibn Abi Shayba 9:678

Abu Bakr ibn Ayyash narrated to us, from Abu Ishaq, from Za'idah ibn Umar, from Ibn Abbas regarding His saying: "Your wives are a tilth for you," he said: "Whoever wishes to practice 'azl (coitus interruptus) may practice it, and whoever wishes not to practice 'azl need not practice it."