Musnad Ahmad 30:274

Sufyan narrated to us, from Isma'il bin Umayyah, from Abu Muhammad bin 'Amr bin Hurayth al-'Udhri—he said once: from Abu 'Amr bin Muhammad bin Hurayth—from his grandfather: I heard Abu Hurayrah saying: Abu al-Qasim (peace and blessings of Allah be upon him) said: "When one of you prays, let him place something in front of his face. If he does not find anything, let him set up a staff. If he does not have a staff with him, let him draw a line, and it will not harm him whatever passes in front of him."