This procedure is normal, especially since they only asked you for the source of the funds and not for you to make more deposits or any fraudulent behavior. Has your account activity in the recent period been different from the normal account behavior?
That's a procedure named as KYC and most sites need KYC verification from the customers. Just send them your KYC information if you don't hesitate much about privacy and after checking your documents and verifying those, they might end up releasing your funds.
In fact, it is an AML (Anti-money laundering) procedure, the purpose of which is to verify that your account is not being used for illegal activities such as money laundering, and it occurs when you engage in low activity. For example, you have had an account in Coinbase for years and have been depositing and withdrawing about 3,000 dollars per month for several years, and suddenly your account appears by receiving average payments of $300,000 per month, which is not in line with the average wage increase, your account is frozen and a source of funds is requested from you.