Sunan Ibn Majah, Chapter: The Book of the Sunnah

"We were with the Prophet (saw), and he drew a line (in the sand), then he drew two lines to its right and two to its left. Then he put his hand on the middle line and said: 'This is the path of Allah.' Then he recited the Verse: And verily, this (i.e. Allah's Commandments) is My straight path, so follow it, and follow not (other) paths, for they will seperate you away from His path..."[Al-An'am 6:153] (Da'if)