How to document cash payments to contractors?

[David Jiang]

What would be sufficient proof that a cash payment I make to a contractor is an expense for a project? I always have trouble with this as I do a lot of flips and have a few people who moonlight for me on their weekends and prefer a cash payment. Is the cash withdrawal showing up in my bank account enough proof or do I need some kind of receipt or invoice from the person I'm paying?