> Are you sure?Yes, absolutely sure.
> To get a mortgage
I said:
>> make your mortgage or car payment
Making payment on existing loan != taking out a new loan.
> they check the source of your down payment and it cannot be cash.
If you mean literal physical cash: the main reason for this is that no one involved in the transaction is interested in taking on the risk of carrying briefcases full of cash across town.
If you mean money in a bank account that is of unknown provenance: This isn't really an issue unless you make a huge lump sum deposit just before closing and have absolutely zero documentation regarding where it came from. Even then, giant sums recently deposited isn't a problem. You just have to prove that it's of non-criminal provenance. Recent pay stubs or sales receipts will do.
That has nothing to do with the money being "cash" or not, though.
And even then, it's mostly about access to giant piles of credit. If you don't need to borrow huge piles of someone else's money, then no one gives a shit. E.g., I've purchased property with a cashier's check, which is about as close to literal cash as you can get without major security risks involved.)