The Poverty of Theory and Other Essays - E.P. Thompson

Does anyone know the source for the quote Thompson attributes to Marx in the first couple pages, "to leave error unrefuted is to encourage intellectual immorality"? That sure would be something if Marx never actually said it... but I guess correcting Thompson's error would have been what he wanted. The quote shows up in this text by Henry Hyndman, where it seems like he is saying Marx had said it? I'm guessing Hyndman is giving an account of an encounter he had with Marx. I looked through Marx's letters and couldn't find it, unless it was in a letter Marx sent to Hyndman which Hyndman lost/destroyed. Hyndman writes,

Hyndman wrote: When George stated that all which was not wages was rent, it seemed incredible that any one should fail to inquire who then takes profit and interest? What had become of them? Therefore, I argued, George will teach more by inculcating error than other men can impart by complete exposition of the truth. Marx would not hear of this as a sound contention. The promulgation of error could never be of any good to the people, that was his view. “To leave error unrefuted is to encourage intellectual immorality. For ten who go farther, a hundred may very easily stop with George, and the danger of this is too great to run.” So far Marx.

Lenin in an article dealing with Hyndman's text seems to have had no doubts and writes (my emphasis),

Lenin wrote: “Marx,” writes Hyndman, “would not hear of this as a sound contention. The promulgation of error could never be of any good to the people, that was his view. ‘To leave error unrefuted is to encourage intellectual immorality. For ten who go farther, a hundred may very easily stop with George, and the danger of this is too great to run!’” That was what Marx said!

Sorry, edited my original post.