If you have not already given them one, they would need a W-9 to issue you a 1099. This basically has your social security number or EIN, name and address for them to issue the 1099.
The only other documentation they would need would be a basic bill/invoice, like a receipt showing how much they paid you and the services you performed. There is not a specific format that must be used. Just make sure it has your name/Business name, contact info and what you did and the amount charged.