I recall some part of the written Torah that refers to the Oral Torah, meaning that we are commanded to pass on the Oral Torah. Which part is it?

The Torah refers in several places to the Oral Law. For example, regarding sh'chita (how to slaughter kosher animals), God tells us "… you may slaughter of your cattle and of your sheep, which the Lord has given you, as I have commanded you,"(Devarim 12, 21) but we don't find anywhere in the Written Torah the details of those commands how to slaughter, so clearly they were oral! Similarly, in Vayikra (26, 46) it refers to the "Torahs" (in plural) which Hashem gave us, referring to the Written and the Oral Torahs. Similarly we are obligated (Devarim 17, 11) to listen and observe what our rabbis teach us (=including the Oral Torah) "According to the law they instruct you and according to the judgment they say to you, you shall do; you shall not divert from the word they tell you, either right or left." Also, there are inborn "difficulties" in the Torah itself which prove that there were additional oral instructions, such as: How do we know what are "Totafot" (Dvarim 6, 8) or "the fruit of the beautiful tree" (Vayikra 23, 40)? It's illogical that God gave many such mitzvot which He wants us to observe, without explaining what they mean.