Musannaf Ibn Abi Shayba 9:700

Katheer bin Hisham narrated to us, from Ja’far bin Burqan, who said: Maimun bin Mihran narrated to us that Ibn Umar bought a slave girl for one of his sons. He said: "Why do I not see her becoming pregnant? Perhaps you are practicing *azl* (coitus interruptus) with her. If I knew that, I would have made your back ache (i.e., punished you)."