There is no law, that is why you and no one else can find one. A legal contract requires specific actions that the parties agree to comply with, it is called acceptance and accord, a check cannot do this. Did your check specify an interest rate for the loan or a required date of repayment? No? Then it's not a contract.
I was told to find the law code that would support this online (which I am unable to locate). Anyone?
There are things that when written on a check may hold the check recipient liable for specific actions. These are called restrictive endorsements. Your statement on the check that it is a loan in itself would not be considered a restrictive endorsement and cannot be held against the recipient.
If you feel my answer is rude, mean, snarky or in anyway not to your liking, I did my job. You don't need to tell me.
No private messages, I do not reply to them.