So my dad brought an X number of dollars (more than $10000 ) when he came to the US. According to my dad , he is positive that he signed some papers declaring this money at the airport at the customs desk.

I am wondering if there is a way that I can contact us customs to find out if I can get a written proof that my dad declared this money, because I want to invest this money and i wanna make sure that all this money is declared