Musnad Ahmad 1060:62

Bihz ibn Asad narrated to us, Hammad—meaning Ibn Salamah—narrated to us, Ishaq—meaning Ibn Abdullah ibn Abi Talhah—narrated to us, from Rafi ibn Ishaq, from Abu Ayyub al-Ansari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not face the Qiblah with your private parts, and do not turn your backs toward it."