Nietzche`s critique of the Cogito

What you're thinking of is probably expressed in chapter 5 of Beyond Good & Evil. The full text is available online, but the passage I'm referring to is:

we really ought to free ourselves from the misleading significance of words! The people on their part may think that cognition is knowing all about things, but the philosopher must say to himself: "When I analyze the process that is expressed in the sentence, 'I think,' I find a whole series of daring assertions, the argumentative proof of which would be difficult, perhaps impossible: for instance, that it is I who think, that there must necessarily be something that thinks, that thinking is an activity and operation on the part of a being who is thought of as a cause, that there is an 'ego,' and finally, that it is already determined what is to be designated by thinking—that I KNOW what thinking is.

This is mainly a point which is, I would argue, wrongfully attributed to the analytical philosophers in the early 20th century: That we forget to question the validity of the intrinsic subject-predicate logic of our language

In Beyond Good and Evil, Nietzsche expresses doubt that the statement "I think" is self-evident, noting that it requires a number of assumptions that Descartes does not justify. Probably the most important of these is that the subject "I" actually exists, such that Descartes is begging the question; another is that if a thought occurs, it is Iwho think it rather than it (the thought itself). In Nietzsche's words:

With regard to the superstition of the logicians, I shall never tire of emphasizing a small terse fact … namely, that a thought comes when “it” wishes, not when “I” wish, so that it is a falsification of the facts of the case to say that the subject “I” is the condition of the predicate “thinks.” It thinks; but that this “it” is precisely the famous old “ego” is … only a supposition,… and assuredly not an “immediate certainty.” After all, one has even gone too far with this “it thinks” – even the “it” contains an interpretation of the process, and does not belong to the process itself. One infers here according to the grammatical habit: “Thinking is an activity; every activity requires an agent; consequently”…. Perhaps some day we shall accustom ourselves, including the logicians, to get along without the little “it” (which is all that is left of the honest little old ego).

Beyond Good and Evil, § 17