In the US, for instance:
> financial institutions covered by the final rule must take reasonable steps to: [...] (3) determine the source(s) of funds deposited into the private banking account and the purpose and expected use of the account;
https://www.fincen.gov/fact-sheet-section-312-usa-patriot-ac...
being key...
> banking institutions;
Source: just finished renewing my AML training
Throwaway for obvious reasons.
Side note: I still can't make sense of why lenders care about the money's source being legitimate but the title company doesn't, per my experience: https://news.ycombinator.com/item?id=28163885
They care that the down payment came from legitimate savings with a documented history, to ensure it’s not from another loan they’re not aware of.
And they care that your reported income is stable (you’re reliably employed with a reasonably predictable income stream) so they can reasonably expect you’ll continue to be able to make monthly payments.
Oh yes it is - that's pretty much the definition of anti money laundering and many many people are involved in trying to figure that.