Musnad Ahmad 30:1996

Abu Ahmad narrated to us, Sufyan narrated to us, from Abdullah ibn Abi Labid, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A prophet from among the prophets used to draw lines in the sand. So whoever’s [method of] drawing matches his [the prophet's] knowledge, then it is his knowledge."