Reference for monoidal bicategory of spans?

Can anyone provide a reference for the fact that given a category C with pullbacks and finite products, you get a monoidal bicategory Span(C)? -- Mike Stay - metaweta@gmail.com http://www.cs.auckland.ac.nz/~mike http://reperiendi.wordpress.com [For admin and other information see: http://www.mta.ca/~cat-dist/ ]