Musnad Ahmad 30:343

'Abd al-Razzaq narrated to us, Ma'mar and al-Thawri narrated to us, from Isma'il ibn Umayyah, from Ibn 'Amr ibn Hurayth, from his father, from Abu Hurayrah, in a marfu' (attributed) manner, he said: "When one of you prays, let him pray towards something. If there is nothing, then a staff. And if there is no staff, then let him draw a line, and then it will not harm him what passes in front of him."