The Gemorah in Chulin Daf 17b shows that there is a source "Min ha'Torah" to show the slaughterer's knife to a Chacham from a Pasuk in Shmuel. The Gemorah goes viter and asks a question, "But didn't we learn that one shows the knife to a Chacham only in order to honor the Chacham?
The Gemorah then ends off, "it is indeed mid'Rabanan, the verse is but an Asmachta".
So l'maseh the checking of the knife is in fact any m'Drabbanan and the pasuk from Shmuel is only an asmachta b'alma.
However in the hava minah of the Gemorah we had asked from where do we know that checking the sacin is "min HaTorah". The gemorah then brings a pasuk from Shmuel. I'm assuming when the gemorah says "Min HaTorah" the kavana is "m'doraisa". However the gemorah brings a pasuk from Navi which perahps we would call "m'divrei kabballah" and is on somewhat of a "lesser" level then "d'oraisa" or "min haTorah". Why then does the gemorah seem to accept this if my assumption of what "min HaTorah" means is true? Do we see that perhaps Navi is on the same level of "Torah" or does the question "min HaTorah" encompass all parts of Tanach and lav davka means what we call "m'doraisa".
Yehoshua, Yerushalayim, Eretz Yisrael
1. The Chidushei ha'Ramban in Kesuvos (110b; printed in the old editions of Chidushei ha'Rashba) cites this Gemara in the middle of a different discussion concerning whether there is an obligation mid'Oraisa to give one's wife a Kesubah. He cites the Gemara in Kesubos (10a) that cites the verse, "k'Mohar ha'Besulos," and then states that "from here the Chachamim 'Samchu' that the Kesuvah is d'Oraisa." The Ramban writes that although in reality the Kesuvah is a Torah obligation, this is in fact a Halachah l'Moshe mi'Sinai like the rest of Torah she'Ba'al Peh. The Ramban then compares this to the Gemara here in Chulin (17b) where the Gemara asks, "How do we know that there is an obligation in the Torah to check the knife?" even though the Gemara proceeds to cite a verse from Divrei Kabalah, i.e. a verse from Navi.
2. The Ramban appears to mean that although this Gemara cites a verse from Navi, the obligation to check the knife is in fact a Halachah l'Moshe mi'Sinai. Hence, if there is a verse in Navi, this is not considered as though there is an explicit verse in the Torah, but rather it indicates that there is a Halachah l'Moshe mi'Sinai on this matter and therefore the Din is d'Oraisa. If there would not have been a Halachah l'Moshe mi'Sinai, there would not have been a hint at this in Navi.
3. I would like to suggest that this is also the intention of the Bahag, cited by the Maharatz Chayus in our Sugya, who writes, "How do we know that the obligation to check the knife is Min ha'Torah? -Since it states, 'And you shall slaughter with this.'" This implies that according to the conclusion of the Gemara as well, the obligation is mid'Oraisa, and in addition this is derived from the verse in Navi. This may mean that it is in fact a Halachah l'Moshe mi'Sinai.
4. The Ya'avetz here also asks your question. He answers that "Nevi'im are referred to as 'Torah,' as in many other places." The Yaavetz perhaps means that although the Gemara calls it Torah, the Navi is also called Torah. Perhaps he is referring to what Rav Ashi says in Rosh Hashanah (19a), that Divrei Kabalah are like Divrei Torah. According to this, although the obligation to check the knife is derived from Navi, this obligation is called Divrei Torah because the Navi is also Torah, but Halcahically-speaking it does not actually have the status of a Din d'Oraisa.
5. The truth is that this question requires a lot more study and is clearly a deep subject. I have attempted merely to show some sources which address the question.
Kol Tuv,
Dovid Bloom