Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
Doesn't say "It is written." does it?
No it doesn't, but the Jews, especially the Pharisees, followed oral tradition. So, just because it wasn't written doesn't mean it's any less biblical. He is obviosuly referring to Lev 19:18 and Deut. 23:3-6 in a very altered fashion, as if there were such a verse.