Comment by jacquesm
If I get the OP correctly it wasn't 'cash' that they were sitting on but it was a balance in a bank account. You can turn that into cash (unless lots of other people try to do the same thing at the same time, see also 'bank run') but strictly speaking it isn't cash.
Nobody is questioning the source of funds for the balance in a bank account that’s been there for forty years.
OP could literally just put it in the bank, sit on it for a year, and there’d be zero questions. They are mostly only interested in knowing that you didn’t get this money from a different loan that they aren’t aware of (like a personal loan, or one you got two days ago that hasn’t yet been reported).