Levítico 23:40 Halacá: Mishneh Torah, Shofar, Sukkah and Lulav, Treasures Hidden in the Sand & Gray Matter III

We do not blow with a shofar of idolaters at the outset. But if one did blow [with one], he has fulfilled [the commandment]. And if one blew [with a shofar] of a condemned city (eer hanidachat), he has not fulfilled [his obligation. Regarding] a stolen shofar with which he has blown, he has fulfilled [the obligation]. For the commandment is only listening to the sound — even though he did not touch it and did not raise it up, the listener has fulfilled [it] — and there is no law of theft with sound. And likewise should one not blow with the shofar of a burnt offering; but if he did blow [with it], he has fulfilled [the obligation]. For there is no law of misappropriating [the sacred] with sound. And if you say, "Did he not derive pleasure from listening to the sound"; [the answer is] that the commandments are not given for pleasure. Hence, one who has vowed not to derive pleasure from the shofar is permitted to blow the blow of the commandment with it.

And this is similar to what Maimonides of blessed memory explained in the introduction to his commentary on the Mishna, that regarding the Ethrog and Lulov we have a tradition from Sinai that they are what is referred to in the verse in the Torah "The fruit of goodly trees and branches of palm trees" (Leviticus 23:40), but, they (the authors of the Mishna) wanted to show carefully how it is derived from the Torah, because all of the Halachot are included in the Torah and therefore in any event, that if an Halachah is not found explicitly in the Torah, it is incumbent that it be derived from the (accepted) modes of derivation from the Torah, see there.

The fruit of goodly trees specified in the Torah is the ethrog. The boughs of leafy trees specified in the Torah refer to the myrtle, the leaves of which cover the wood; it has, for example, three or more leaves growing from each bud. However if two leaves were one across from the other, but the third leaf was above them, this is not [considered] "leafy." Rather it is called a fool's myrtle