Full Idea
Kripke's argument against mind-brain identity is that a pain is necessarily pain (just as a stone is necessarily matter), but a brain state is not necessarily painful (just as a stone is not necessarily a doorstep).
Gist of Idea
Kripke says pain is necessarily pain, but a brain state isn't necessarily painful
Source
report of Saul A. Kripke (Naming and Necessity lectures [1970]) by Georges Rey - Contemporary Philosophy of Mind 11.6.2
Book Reference
Rey,Georges: 'Contemporary Philosophy of Mind' [Blackwell 1997], p.308
A Reaction
As with Descartes' argument from necessity for dualism, this seems to me to beg the question. It seems to me fairly self-evident that certain brain states have to be painful, just as stones always have to be hard or massive.