If you paid the premiums with tax-free money (such as from an employer pre-tax payroll deduction) then the rebate is now taxable income to you. If you paid the premiums yourself for private or marketplace insurance and took the schedule A itemized deduction for the premiums, then this may also be taxable income if it is a reimbursement of a previous deduction.
You must apply the "tax benefit rule". For example, suppose you paid $5000 in premiums, and this was less 7.5% of your income so you did not actually get a deduction. The rebate is not taxable since it is not a return of a previous deduction. Or, suppose you paid $12,000 in premiums and 7.5% of your income was $11,000, so you got a $1000 deduction. If the rebate was $1500, the taxable amount is only $1000 since that is the tax benefit you received. If you received a premium tax credit, you will have to refigure the PTC for the year you paid the premiums to see if any part of the rebate is taxable. Taxable recoveries are reported as "other less common income" at the bottom of the income page.
Once you resolve the situation of whether the rebate is taxable or not, then the contribution is handled as a separate transaction according to its own rules. Once the money is in your pocket or bank account, it mingles with all your other money and there is no difference where the HSA contribution came from. (This, of course, assumes you are eligible to make contributions to an HSA.)
