Is a distribution received in 2019 of ineligible HSA salary reduction contributions made in 2018 reported in 2018 or in 2019?

In 2018, I contributed to a health savings account through pre-tax salary reduction contributions.  I recently learned that my wife enrolled in an FSA in May, 2018, making me HSA-ineligible.  I have asked for a return of the $2,000 in HSA contributions that were made after April of 2018.  How do I report the $2,000?  Is it reported on may 2018 return?  Or is is reported in 2019 because that is the year in which the amount will be received?