Midrash Tannaim 6.7, Sifre Deut. 34 (in MSS.)
ודברת בם עשם עקר ואל תעשם טפלה שלא יהא משאך ומתנך אלא בהם שלא תערב בהם דברים אחרים כפלוני שמא תאמר למדתי חכמת ישראל אלך אלמד חכמת האומות תל ללכת בהם (ויקרא י"ח ד') ולא ליפטר מתוכן
"And thou shalt speak of them: Make them thy main concern, and not ancillary to other things; so that thy discussions will be only about them, and thou shalt not mix other things with them, like Peloni*. Perhaps thou maysest say, I have studied the wisdom of Israel, I will now go and study the wisdom of the nations, therefore doth Scripture say, To go in them (Lev. 18:4), and not to turn away from them."
In JBL 53:2 R. Louis Finkelstein asked a very good question: who is meant here by פלוני, Peloni?
Some have suggested it referred to Aher, but Finkelstein rejects this because 'Aher' is itself a euphemism. Where do we find him called Peloni? Secondly, we do not actually find a decription of him as a Greek philosopher in the Talmud (despite popular conceptions).
We find him trying to get Jewish children to stop learning Torah (B. Hagigah 15b), reporting Jews who tried to minimize their hillul shabbat which was required by Roman decree (Jer. Hagigah ch. II). These depictions portray him as one who collaborated with the Romans. The story of him entering the pardes doesn't impy his engagement with Greek philosophy, because R. Akiva and others did the same. The Gemara does say that he carried סיפרי מינין, but מין refers to Jewish heretics, not the Greek philosophers.
Then who is intended by Peloni?
The answer might be that כפלוני, like Peloni, is a mistake that should have read כפילוני or כפילון, "like Philo." The passage becomes even more clear if this is so, because it is not discussing an apostate--Philo was certainly no such thing. If so, then this midrash disapproves of this person because he was a Torah u-Maddanik, not mamash a heretic! :)
R. Finkelstein tentatively suggests a couple of other references to Philo in rabbinic literature, of which he is less certain, but this one is cleaner.
*In rabbinic literature Peloni is used for "so-and-so." The origin is from Ruth 4:1, where פלני אלמני, meaning "such a one" ie, so-and-so is used.