I believe it's Reg X 1024.41(g)
g) Prohibition on foreclosure sale. If a borrower submits a complete loss mitigation application after a servicer has made the first notice or filing required by applicable law for any judicial or non-judicial foreclosure process but more than 37 days before a foreclosure sale, a servicer shall not move for foreclosure judgment or order of sale, or conduct a foreclosure sale, unless:
(1) The servicer has sent the borrower a notice pursuant to paragraph (c)(1)(ii) of this section that the borrower is not eligible for any loss mitigation option and the appeal process in paragraph (h) of this section is not applicable, the borrower has not requested an appeal within the applicable time period for requesting an appeal, or the borrower's appeal has been denied;
(2) The borrower rejects all loss mitigation options offered by the servicer; or
(3) The borrower fails to perform under an agreement on a loss mitigation option.
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CRCM
"Do or do not ... There is no try"