Years ago I heard a piece by Haydn that had an introduction or 1st movement, or whatever the beginning is called (I`m not a classical music buff)by Mozart. In other words it`s a symphony or something BY Haydn, but with the beginning done by Mozart. A collaberation I guess. Anyway, does anyone know which work this is????? I loved it but haven`t heard in years.

Hayden on a Theme by Mozart
start looking with that region of name, you`ll probably find something of the sort.
Also, try the Carnegie online library, they have a whole collection of pieces that may be of your interest, and you might find the piece you`re looking for.
good luck!

Thanks Jarvinia!
And I might be wrong about Haydn, might have been Handel.

can`t have been Handel as he was earlier than Mozart. but your on the right lines of Haydn