Source of wealth document are what is supposed to be requested for in banks or an investment company as a document to shows where you actually got the money you intended to either save in bank or invest in an investment company. However, you alone at O.P knows where you got your money from, because I'm pretty sure for this casino to have requested for a Proof of Source of Wealth document, it's likely the amount must be huge, because such documents aren't requested for small amounts.  So a salary pay cheque could be a Source of Wealth document, and likewise a receipt for a sold item can also be a proof of source of wealth document.