The obligation to demand proof of origin of funds is precisely defined in the German Money Laundering Act (as one example for the legal situation in the European Union). In everyday life, the following situations are currently likely to be of particular importance: Depositing more than EUR 10,000 of cash at your bank into your current account, Deposit or withdrawal of fiat money at your cryptocurrency exchange, Cash purchase of precious metals worth more than EUR 10,000 at your local bank or more than 2,500 EUR at a credit institution that is not your local bank. There are numerous other situations where proof of source of funds is required, e.g. real estate transactions. The above limits may change over time.