Source of funds means the origins of your income, your wallet is the balance sheet!
Stake
is pretty clear on this, the source of funds means a document attesting you have received those funds in a lawful way, a simple bank statement that doesn't show deposits which could be labeled as constant payment from being either an employee or a freelancer will not suffice either.
AML regulations are not different from crypto, and your crypto wallet would hold as much value as a paysafecard account.
For example, exchange account history that contains trading and deposit history, if well documented, can be used as a source of funds since there will be a proper record of how trades were carried out, a significant profit was made, and the time frame that it took for that to happen.
So I buy some Bitcoin from an ATM with cash, I send it to Binance, withdraw to my bank, I get a statement and suddenly I'm all ok and all my funds are clean?