Musannaf Ibn Abi Shayba 3:468

Ibn ‘Ulayyah narrated to us, from Yunus, from Humayd ibn Hilal, from ‘Abdullah ibn al-Samit, from Abu Dharr, who said: The Prophet (peace and blessings of Allah be upon him) said: "When one of you stands to pray, he is shielded if there is in front of him [something] like the back of a saddle."