How do I report a hackers email to Yahoo?

I received an email supposedly from a contact of mine, but it was a web address for Viagra. All 3 of my email addresses received it, and two Yahoo groups which both myself and my friend are in. In fact she is the moderator in one of these groups. Its obviously been hacked. Where do I go to report the full header details to Yahoo? It was her Yahoo email address which got hacked,which is how it reached the Yahoo Groups as well.