529 Qualified Withdrawal Time

Can anyone cite the IRS source that states a 529 qualified withdrawal must be in the same calendar year as when the tuition was paid?  I would like to make a qualified withdrawal for last year's K-12 qualifying tuition.

 

I have seen many responses claiming the calendar year requirement as a fact, but am unable to locate the source document stating such a rule...including this thread:  https://ttlc.intuit.com/community/college-education/discussion/reimbursement-from-529-plan/00/327168...

 

Thanks