I think that we can agree that the Best source will be the receipt. However, I believe that during audit or appeals you sometimes get the wiggle room and statements and Credit card receipts has giving some allowance. But do not put your hope in this, Trends would have to be made and with some statements the argument can be made.
Example: The Credit card statement say $125.00 to California Real Estate board, If the person is a licensed RE then it is not to hard to use this to demonstrate the expense for their license and might be accepted.
The gold standard is the reciept, not even the credit card slip. Since the reciept has information as to what was purchased. The next useable proof is the statement,however this does not tell you what was purchased and as the taxpayer you have the burden of proof.
Example, you visit staples and purchase a $1000 computer, without the receipt the statement will only say staples$1000. How can you prove that you got a computer or $1000 in gift cards and gave them to family for Christmas (Yes this has been done).