What expressed power did the Supreme Court cite when it upheld the constitutionality of the Affordable Care Act?

The expressed power the Supreme Court cited when it upheld the constitutionality of the Affordable Care Act (ACA) was: the power to tax.

Explanation

Even though the Supreme Court declared that the law could never be upheld, by a decision based on the regulatory power of Congress, the Court ended accepting that it was constitutional and valid for the Congressional power to tax, as a consequence, defending the individual mandate.