Where is this quote from? (Marx and Engels)

[JKP]

This is from a Solzhenitsyn speech, so keep that in mind.

"In their correspondence Marx and Engels frequently said that after achieving power, terror would be indispensable, that "it will be necessary to repeat the year 1793. After achieving power, we'll be considered monsters, but we couldn't care less" (vol. 25, p. 187). "

[Sun at Eight]

It doesn't turn up in any searches of Marx/Engel Correspondence on Marxists Internet Archive nor on the extensive German collection of MLWerke, as far as I can tell. When I searched the quotation on Google I got the Solzhenitsyn speech where it appears on Google Books and the key part is that the volume and page references are to the "first Soviet edition of 1929". This may refer to the German-language edition prior to the current German edition started in East Germany in the '60s. I think the only way to find it would be the search for "1793" (which is what I did) since we're be talking about a non-academic citation in a speech and that quotation may be translated from German to Russian to English, if it indeed exists. Does anyone have the table of contents of the "first Soviet edition of 1929"?