Limud Torah

The Priah is a part of the Milah. The Mishna says that if you do the Milah without Priah, it's as if you didn't do the Milah. There is no mention of it in the P'sukim which talks about the Mitzvah by Avrohom Aveinu. This is why the Gemara Yevamos 71a says that Priah was not given to Avrohom Aveinu. We only see it written in Sefer Yehoshua. Even though after the Torah, no Navi can come along and add Halachos to the Torah (so how can we learn the Mitzvah of Priah from Yehoshua?) Tosfos answers, it was really given as part of the Halachos L'moshe MiSinai. When Yehoshua wrote his Sefer, he incorporated this Halacha MiSinai as a Pasuk. Therefore, Yeshua was just quoting a Halacha that was given orally at Sinai.