It depends. It's customary to supply a receipt with a charitable donation of any decent size for which you're claiming a tax rebate, but as far as I know, it's not actually legally required.

If you get audited and you don't have that receipt, you're up shit creek. But as we've seen the last few years, the IRS doesn't have the balls to go up against rich people's accountants and lawyers to get the tax revenue they're dodging — they'd rather raid some little mom-and-pop donut shop to demand receipts for bulk toilet paper.