It's in the Internal Revenue Code, CFR 1.408-8 Q&A-4: <a rel="nofollow" target="_blank" href="
https://www.law.cornell.edu/cfr/text/26/1.408-8">https://www.law.cornell.edu/cfr/text/26/1.408-8</a>
"an amount distributed during a calendar year from an IRA is treated as a required minimum distribution under section 401(a)(9) to the extent that the total required minimum distribution for the year under section 401(a)(9) for that IRA has not been satisfied."
In other words, until the RMD for the year for the particular IRA has been satisfied, distributions from the IRA are RMDs. RMDs are not eligible for rollover.
