If you withdraw money from your HSA (Health Savings Account), you will receive tax Form 1099-SA. When you enter that on your tax return, you will be asked if you used the money to pay for qualified medical expenses. If you did, there is no tax or penalty. If you answer NO and you did not use the money for qualified medical expenses, you will pay ordinary income tax + a 20% penalty when you file your tax return. You don't pay the penalty until you prepare your tax return.
If my answer is that I used that money for medical expenses, I must have all the supporting documents for those medical expenses, is that correct?
"I must have all the supporting documents for those medical expenses, is that correct?"
Of course. If you are ever audited, and your HSA is part of the audit, then I would presume that the IRS auditor would review your tax returns along with the paperwork of the HSA custodian.
By the way, you need to collect those supporting documents now. IRS agents don't take papers thrown together at the last minute very seriously.