You don't send proof to the IRS when you file. Save your proof of disability with your other important papers in case of audit.
For income taxes, disabled means unable to perform gainful work due to a condition that is permanent. There is no specific proof specified in the law, although there is a general principle that the burden of proving the right to any deduction or adjustment is on the taxpayer. While a certification from the SSA would almost certainly be accepted, a strong letter from your doctor should be enough even if the SSA is still working on it.
