How do I prove we gifted cash money to my in laws?

[VolvoGirl]

You don't report gifts given or received on your tax return.  Why do you need to prove it and to who?  Probably only your bank statements have that info.  

[SweetieJean]

If someone (say a Lender) needs that, ask them what they will accept as proof.

[SweetieJean]

The recipient of a gift can record it as nontaxable income for purposes of the Sales Tax Deduction if they itemize.

[Coleen3]

You don't have to. You are allowed a gift of up to $15,000 per year, per person. If you gift more than that to any one person, you will file a Form 709, Gift Tax Return.

https://www.irs.gov/pub/irs-pdf/i709.pdf