Parshat ראה is the source for ritual slaughtering. It is learned from the words וזבחת כאשר צויתיך, that you shall slaughter as I commanded you. From these words, “As I commanded you,” we have an allusion to the Oral Law.
Such well known words in Jewish life as, שחיטה, תפילין, גט, and אתרוג, are understood even by Jews with a minimal background. Yet, these four words do not appear anywhere in the Torah.
The fact that practicing Jews know what these words mean, is a proof that the Oral Law works hand in hand with the Written Law.
Both were given on Mount Sinai and both have the strength of commandments with Hashem’s full endorsement. Just like we are not allowed to challenge the Written Law, we are equally obliged to observe the Oral Law.