From the portion of the Torah read last week (Shmini):
Vayikra/Leviticus 11:26-27
26. Any animal that has a cloven hoof that is not completely split, and which does not bring up its cud, is unclean for you. Anyone who touches them shall become unclean.
27. And among all the animals that walk on four legs, any [animal] that walks on its paws is unclean for you. Anyone who touches their carcass will be unclean until evening.