Musannaf Ibn Abi Shayba 3:6467

Abu Bakr narrated to us, he said: Sufyan ibn 'Uyaynah narrated to us, from Isma'il ibn Umayyah, from Abu Umayyah Muhammad ibn 'Amr ibn Hurayth, from his grandfather, that he heard Abu Hurayrah saying: "When one of you prays in an open land, let him set up a staff. If he does not have a staff with him, let him draw a line on the ground, and it will not harm him what passes in front of him." Abu al-Qasim said: He means his beasts of burden.