OK, the title is opinionated and contentious, but I have a definite question. I know that the title refers to the Bourbaki volume Groupes et Algèbres de Lie (Chapters 4-6), published in 1968, but

who said that it is the only great book that Bourbaki ever wrote?

The only reference I can find is the 2009 Prize Booklet for the AMS-MAA Joint Meetings, where no source is given, but I'm sure I've seen the claim somewhere else.

Edit. I have rolled back the title of this question to almost its original form, because putting the title in quotes misled some people into thinking I sought a source for the exact phrase "the only great book that Bourbaki ever wrote." Rather, I wanted a source (not necessarily unique) for the idea that Chapters 4-6 of Groupes et Algèbres de Lie is Bourbaki's one great book. Gerald's answer and Jim's comment together are exactly what I wanted.