Is the following quote by Appius Claudius Caecus or by the Emperor Claudius: "Do not say all that you know, but always know what you say"

Well, like many quotations, this one is hard to pin down. Many websites attribute it to Appius Claudius, while a number of books available in Google Books attribute it to simply "Claudius," presumably meaning the Emperor, more often written in that case, as "Say not always what you know, but always know what you say." However, I've searched a number of books of specifically Latin or Classical quotations/aphorisms, and none of them have the quote, even the ones that include other sayings of Appius Claudius. So, I am skeptical about it originating with either of them.

There are other sources that attribute it to Matthias Claudius, a German poet and there is an oft-quoted German version: "Sage nicht alles, was du weißt, aber wisse immer, was du sagst." However, I am again skeptical of this attribution since I find it only in books of sayings, sometimes attributed again to "Claudius," sometimes to Matthias Claudius, but without any specific citation or context, which should be the case with an 18th/19th century poet.

The most reliable attribution for a similar quote I could find was to Imam Ali (son-in-law of Muhammad), one of whose sayings is translated as something like "Do not say what you do not know, but do not say everything you know." This is sometimes given in the context of a longer quote so is more reliable than other citations. Of course, it is possible it was a familiar expression at the time and may have come to him from classical sources.