David Yaffe puts there is 'no final crisis' in quotes as if Marx said it but gives no reference (because he was doing it from memory?) in his articles on crisis in Science an Society and Revolutionary Communist. The quote is usually introduced in the argument where the TRPF has been set out and the author is about to move on to discuss the coutneracting tendencies that stop the falling rate of profit from being a permanent crisis. The nearest to it in Marx is a passage in the Theories of Surplus Value, except that what he says is more or less the opposite in its conclusion:
"These contradictions of course lead to explosions crisi in which momentary suspension of all labour and annihilation of a great part of the capital violently lead it back to the point where it is enabled [to go on] fully employing its productive powers without committing suicide. Yet these regularly occurring catastrophes lead to their repetition on a higher scale, and finally to its violent overthrow." Grundrisse p 750, Penguin 1973
Marx does say 'permanent crises do not exist'. TSV, II, p 497, fn, Lawrence and Wishart, 1969