Show that $\displaystyle 1 - i$ is an irreducible in $\displaystyle Z[i]$.

Proof. Suppose that 1 - i = ab, for a and b in Z[i], we need to show that either a or b is a unit.

I'm having trouble trying to do this one, is there a certain factor that 1 - i has that can allow me to conclude the other has to be a unit?