I don't think the IRS has a clarifying document on this, but it's written into the law
https://www.law.cornell.edu/uscode/text/26/129
26 U.S. Code § 129 - Dependent care assistance programs
(b) Earned income limitation
(1) In general
The amount excluded from the income of an employee under subsection (a) for any taxable year shall not exceed—
(A) in the case of an employee who is not married at the close of such taxable year, the earned income of such employee for such taxable year, or
(B) in the case of an employee who is married at the close of such taxable year, the lesser of—
(i) the earned income of such employee for such taxable year, or
(ii) the earned income of the spouse of such employee for such taxable year.
