Halakha Imamat 20:25: Mishneh Torah, Forbidden Foods, Shulchan Arukh, Orach Chayim, Sefer HaMitzvot & Sefer HaChinukh

It is a mitzvah to discern the distinguishing marks of animals that are allowed and forbidden to eat — whether cattle, game, birds, fish and locusts — as is written: "Ye shall therefore separate between the clean beast and the unclean, and between the unclean fowl and the clean" (Lev. 20:26); "to make a difference between the unclean and the clean, and between the living thing that may be eaten and the living thing that may not be eaten" (Lev. 11:47).

If, while in the middle of one's prayer [i.e. Amidah], he feels an urge [to relieve himself], he should maintain himself [in prayer] until he has finished, and not interrupt. And if at the time of the Recitation of the Sh'ma and its blessings he feels an urge, regardless of whether it is for urinating or defecating, he may read it normally. Rem"a: And [this is] specifically if one does not feel so much of a urge that there is an element of "Do not cause an abomination" (Leviticus 20:25). But if this is not so, it is better to interrupt [one's prayer and use the toilet] (Responsum of the Rashb"a, section 131, who ruled that it is permissible, and the Terumat haDeshen section 16, who ruled that it is prohibited. And one needs to distinguish such [i.e. between those sources]) And if one wanted to distance himself and urinate [during the Recitation of the Sh'ma and its blessings], he may.

That is that He commanded us to check for the signs of [permissible] fish, which are written [in the Torah]. And that is His, may He be blessed, saying, "These are what you may eat from all that is in the water" (Leviticus 11:8). And in explanation, they said in the Gemara (Chullin 66b), "One who eats an impure fish transgresses a positive and a negative commandment." As its statement - "These are what you may eat" - [makes] me understand that one besides these may not be eaten. And a negative commandment that is derived from a positive commandment is a positive commandment. Behold it has been made clear that its statement, "These are what you may eat," is a positive commandment. And the content of our saying that this is a positive commandment is what we have mentioned to you; and that is our being commanded to administer these signs, and to say that this is permissible to eat and this is not permissible to eat - as it is stated (Leviticus 20:25), "And you shall distinguish the clean beast from the unclean." And their distinguishment is though the signs. And therefore each and every one of these four types and their signs is a separate commandment - meaning the signs of the beasts and the animals, the birds, the locusts and the fish. And we have already explained their expressions in each of the verses as an individual commandment. And the regulations of this commandment - meaning the signs of the fish - have already been explained in Chapter 3 of Tractate Chullin. (See Parashat Shemini; Mishneh Torah, Forbidden Foods 1.)