halacha - What is the source that Parshas Parah is from the Torah?

I have read in many places that Parah could be a bliblical requirement to read.

Why is that? Zachor makes sense (as it says so in the Torah here). Why Parah?

Answer

The Beit Yosef in OC 685 quotes Tosfot who says that Parashiyot Zachor and Parah are Biblical requirements. However, our versions of Tosfot do not have anywhere that Parah is Biblical. The Mishna Berura OC 685 sk 15 writes that most Achronim agree that Parah is not a Biblical requirement. So we really don't know what source Tosfot had in mind (assuming the Beit Yosef's version didn't have a scribal error, which is possible).

EDIT: The Aruch HaShulchan (OC 685:7) suggests the following possibility for the source. In Parashat Parah the pasuk says (Numbers 19:10):

וְהָיְתָה לִבְנֵי יִשְׂרָאֵל וְלַגֵּר הַגָּר בְּתוֹכָם לְחֻקַּת עוֹלָם
...and it shall be unto the children of Israel, and unto the stranger that sojourneth among them, for a statute for ever.

The sifra derives from here that the ashes remain effective even nowadays without the Temple. However, later on the pasuk repeats (Numbers 19:21):

וְהָיְתָה לָהֶם לְחֻקַּת עוֹלָם
And it shall be a perpetual statute unto them.

The Aruch HaShulchan posits that this extra occurrence of the phrase comes to teach that one should be involved even without the ashes by learning these verses which we do by reading them publicly once a year.

Again, this is all speculation because we don't have an original derivation (as I explained above).