What documents do I need to write off labor for a flip?

[Russell Quealy]

Hello BP!

I've been doing some research and I cant find the answer to my question so I'm coming to you! Right now I'm in the middle of a Fix and Flip and I'm trying to make sure all my documentation is in order for next years tax season! But I had some contractors come to the house and do some light work but it was all paid under the table in cash (about 1k). I didn't think about it at the time but is there anything i need from them like an invoice to be able to write off that labor charge? Or will the IRS just take my word for how much my total labor charge is?  

Also, on the topic, I keep records of mileage, gas receipts, materials, my purchase price, closing costs, basically everything i have spent has been recorded but don't have a physical receipt for everything. 

Any help would be greatly appreciated! 

Thank you!