What is the original Latin for Luck is what happens when preparation meets opportunity?

This quotation is universally attributed to the Roman pholosopher and dramatist Seneca, but nowhere is the specific source mentioned, so it's difficult to say how it was Worded in the original Latin. Until such time as the correct answer can be provided, here's a translation that's almost certainly notthe way Seneca said it:

Fortuna est quae fit cum praeparatio in occasionem incidit.