What is the correct saying? In Zhuang Zi 5.2 we read:

聞之曰：『鑑明則塵垢不止，止則不明也。久與賢人處，則無過。』

Lin Yutang translation: "Now I have heard that if a mirror is perfectly bright, dust will not collect on it, and that if it does, the mirror is no longer bright. He who associates for long with the wise should be without fault." (Notice in the link above how Yutang avoids the quotes.)

I wonder if the saying is both phrases,

『鑑明則塵垢不止，止則不明也。久與賢人處，則無過。』

or only the first one (being the end quote misplaced somewhere in the copy chain).

『鑑明則塵垢不止，止則不明也。』

Looks to me like both phrases make more sense in the context of this book. In some places it is referred in the shorter version, like here and here.
Honestly, it doesn't matter. The "quote" is mostly made up in such context to emphasize the 2nd phrase.