Proof of q-series identity and partitions using ferrer diagrams

I unfortunately missed my class on Friday, and we went over a proof which is not in our text. I know I've seen it before, but I can't remember how it goes.

It's a proof of the identity http://mathworld.wolfram.com/images/...P/Inline34.gif = http://mathworld.wolfram.com/images/...P/Inline37.gif (The equals is supposed to relate the product with the sum, if it isn't obvious. It looks kind of strange how it's written.) This identity is used in finding the number of partitions of an integer.

There is a proof which uses Ferrer diagram's. Does anyone know where I can find this proof, in either a text or online somewhere? I need to know it for an exam coming up on Friday.

Thank you in advance!