One of the most famous Rambans in the Torah is on Vayikra 19-2, where the Torah says “You Shall be Holy”. Here is translation of that Ramban from Sefaria.
You shall be holy: “One should be separate from sexual transgressions and from sin, for any place that one finds a fence [before] sexual transgressions, one [also] finds holiness (kedusha)” – this is the language of Rashi.
But in Sifra, Kedoshim, Section 1, Chapter 2, I saw only, “You shall be holy.” And [so,] they learned there (Sifra, Shemini, Chapter 12:3), “‘And you shall sanctify yourselves and be holy, since holy am I’ (Leviticus 11:44) – Just like I am holy, you should be holy; just like I am separate, you should be separate.”
But according to my opinion, this separation is not to separate from sexual transgressions, like the words of the rabbi (Rashi). But [rather], the separation is the one mentioned in every place in the Talmud where its [practitioners] are called those that have separated themselves (perushim).
And the matter is [that] the Torah prohibited sexual transgressions and forbidden foods, and permitted sexual relations between husband and wife and the eating of meat and [the drinking of] wine. If so, a desirous person will find a place to be lecherous with his wife or his many wives, or to be among the guzzlers of wine and the gluttons of meat. He will speak as he pleases about all the vulgarities, the prohibition of which is not mentioned in the Torah. And behold, he would be a scoundrel with the permission of the Torah.
Therefore, Scripture came, after it specified the prohibitions that it completely forbade, and commanded a more general [rule] – that we should be separated from [indulgence of] those things that are permissible: He should minimize sexual relations, like the matter that they stated (Berakhot 22a), “That Torah scholars should not be found with their wives [constantly] like chickens.” And he should only have relations according to the need for his execution of the commandment.
And he should sanctify himself from wine by minimizing it – just as Scripture calls the Nazarite, holy (Numbers 6:5); and mentions the evil that comes from it in the Torah with Noach (Genesis 9:21) and with Lot (Genesis 19:33).
And so [too], he should separate himself from impurity – even though we are not prohibited from it in the Torah – as they mentioned (Chagigah 18b), “The clothing of ignorant people are [considered] midras (a type of impurity) for perushim.” And just as the Nazarite is also called holy for his guarding [himself] from the impurity of the dead.
And he should also guard his mouth and his tongue from becoming defiled from the multitude of coarse food and from disgusting speech, as mentioned by Scripture (Isaiah 9:16), “and every mouth speaks a vulgarity.” And he should sanctify himself with this, until he comes to separation (perishut) – as they said about Rabbi Chiya, that he never spoke idle conversation in his life.
For these [things] and similar to them comes this general commandment – after it listed all of the sins that are completely forbidden – until he includes in this general rule the command of cleanliness of his hands and his body. As they stated (Berakhot 53b), “‘And you shall sanctify yourselves’ – these are the first waters (to wash hands before the meal), ‘and be holy’ – these are the last waters (to wash hands after the meal), ‘since holy’ – this is fragrant oil (to ward off bad odors).”
As even though these commandments are rabbinic, the essence of Scripture prohibits things like these; that we should be clean and pure and separate ourselves from the masses of people, who dirty themselves with those things that are permissible and with those things that are ugly. And this is the way of the Torah to state the particulars and [then] the general rules.
And similar to this is when after the prohibition of the specific laws of trade among men – do not steal, do not burglarize, do not deceive, and all of the other prohibitions – it states the general principle, “And you shall do the straight and the good” (Deuteronomy 6:18), so that it places into a positive commandment, uprightness, compromise and going beyond the letter of the law towards the will of his friend – as I will explain (there), when I get to its place, with the will of the Holy One, blessed be He.
And so [too] with the matter of the Shabbat, it forbade the types of work with a negative commandment and the exertions with a general positive commandment, as it states, “rest”; and I will explain this more (Ramban on Leviticus 23:24), with the will of God. And the explanation of the verse saying, “since Holy am I the Lord, your God,” is to say that we merit to cling to Him by our being holy.
And this is like the matter of the first statement in the ten statements (Ten Commandments – Exodus 20:3). And it commanded (Leviticus 19:3), “A man, his mother and father must fear” [here], since there (in the Ten Commandments), it commanded about honor, and here it will command about fear. And it said [here] “and guard my Shabbats,” since there it commanded on the remembering [of the Shabbat] and here on the guarding and we have already explained the matter of both of them (see Ramban on Exodus 20:8).