Musnad Ahmad 27:1668

Ubaydah ibn Humayd narrated to us, Ubaydullah ibn Umar narrated to me, from Nafi’, from Ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray, and he would place a camel between himself and the Qiblah." And Ubaydullah said: I asked Nafi’, and I said: "When the camels were gone, what did Ibn Umar use to do?" He said: "He used to place the back part of the saddle between himself and the Qiblah."