In the acknowledgments to the collection The Birth to Presence, Nancy confesses that he has not always found it possible to provide bibliographical references for his quotations: “Some readers may take this to be oversight or a blameworthy hastiness, even if the reference is to a well-known text. (‘What is “well-known” isn’t known at all,’ writes Hegel; I know this sentence well, but I don’t know where to locate it in the Phenomenology of Mind.)”
For various reasons, this confession has always stuck out in my mind — it is a reminder of the greater fussiness of English publishers with regard to quotations, and the irony of the specific quotation in question is of course striking. Hence I believe that during the course of my year-long tutorial over the Phenomenology, I would have noticed the quotation if it actually appeared. And my evidence for this bold claim is that I did in fact notice it when I came across it in Addition 2 to paragarph 24 of the Encyclopedia Logic (pg. 59 in the Hackett edition):
In this way the Logic is the all-animating spirit of all sciences, and the thought-determinations contained in the Logic are the pure spirits; they are what is most inward, but, at the same time, they are always on our lips, and consequently they seem to be something thoroughly well known. But what is well known in this manner is usually what is most unknown.
What do you think, readers? Is this most likely the passage Nancy had in mind, or is there a closer match elsewhere?
An und für sich 
Preface to Hegel’s Phenomenology of Spirit (Miller #31, p. 18; Baillie, p. 92; Kaufmann, p. 48): “What is familiarly known is not really known, precisely because it is so familiar.”
Curses! I was thrown off by the slightly different wording! Thanks!