And the 90th prohibition is that we are forbidden from slaughtering any sacrifice outside [the courtyard of the Temple]. This prohibited action is called shochet bachutz.
In the beginning1 of tractate Keritut, where all those who are punishable by karet are listed, the one who does the slaughtering and the one who does the offering2 [on the altar] are listed separately as two [commandments].
The3 punishment of karet for one who just slaughters an animal outside, even if he didn't bring it as an offering, is stated by Torah itself in G‑d's statement4 (exalted be He), "[Any member of the family of Israel] who slaughters an ox, sheep or goat, whether in the camp or outside the camp, and does not bring it into the Communion Tent to be offered as a sacrifice to G‑d, blood shall be counted for that man; he has spilled blood and shall be cut off (spiritually)." The actual prohibition for one who slaughters a sacrifice outside, however, is not written explicitly. It is derived from the principle,5 "Scripture gives a punishment only when there is a prohibition," in accordance with the principles we established in the Introduction to these commandments.6
In the words of the Talmud in tractate Zevachim,7 "One who does the slaughtering and the offering outside is guilty both for the slaughtering and the offering. The case of the offering is clear because both the punishment and the prohibition are written. The punishment is written in the verse,8 'If he does not bring it into the Communion Tent ... he shall be cut off,' and the prohibition is written in the verse9 "Be careful [hishamer] not to bring your burnt-offerings [in just any place that you may see fit]." This is in accordance with Rav Avin, as Rav Avin said in the name of Rav Laya,10 'Every case where the Torah writes hishamer (be careful), pen (lest) or al (do not) indicates a prohibition.' For slaughtering, however, the punishment is clearly written in the verse, 'If he does not bring it into the Communion Tent ... he shall be cut off.' But what is the source for the prohibition?"
After lengthy and wide-ranging discussion, the conclusion is given as follows: "It is written,11 'There shall you offer ... and there shall you prepare.' This creates a comparison between offering and preparing: just as offering has both a punishment and a prohibition, so too preparing has both a punishment and a prohibition." Their reference to, "There shall you offer ... and there shall you prepare," is to G‑d's statement12 (exalted be He), "There shall you offer your burnt-offerings," which refers to burning them on the fire [of the altar]. "There shall you prepare all [the offerings] I have commanded you," includes both the offering and the slaughtering, since He also commanded the slaughtering.
You should be aware that one who slaughters outside unintentionally is also required to bring a sin-offering. You should also be aware that one who offers sacrifices outside the place of the Temple courtyard even now [when there is no Temple] is punishable by karet. Our Sages say explicitly,13 "Rabbi Yochanan says, 'One who brings an offering even now is guilty.'" This is the final ruling, since the animal is actually fit to sacrifice, in accordance with our accepted principle,14 "Sacrifices may be brought even if the Temple is not built."
The details of this mitzvah have been explained in the 13th chapter of tractate Zevachim.