If $G$ is a finite group and $(ab)^3= a^3b^3$, [duplicate]

Possible Duplicate:
Group with an endomorphism that is “almost” abelian is abelian

If $G$ is a finite group and $(ab)^3= a^3b^3$, and $3 \nmid o(G)$, then how do I prove that $G$ is abelian?