source of funds after parlay hit

Had a decent parlay hit recently and now the site is asking for "source of funds" before they’ll process my withdrawal. Never had that come up when playing blackjack or roulette, only with sports bets. I don’t keep receipts for every little bank transfer or cash deposit into my account, so I’m not sure what to send them. My day job isn’t anything crazy and there’s nothing sketchy about my deposits, just feels like a weird hassle after winning bigger than usual.

For anyone who’s had to go through this, what kind of stuff did you actually send over that worked? Not looking to get locked out over paperwork.