Rashi on Leviticus 18:30/רש"י על ויקרא י״ח:ל׳

1ושמרתם את משמרתי THEREFORE SHALL YE KEEP (OR, WATCH OVER) MY CHARGE — This is intended to caution the court regarding this (Sifra, Acharei Mot, Chapter 13 22).

2ולא תטמאו בהם אני ה' אלהיכם AND IF YE DEFILE NOT YOURSELVES THEREIN, I AM THE LORD YOUR GOD — If, however, you do defile yourselves I shall no longer be your God (אלהיכם) since you have cut yourselves off from following after Me. What use then can I have of you? Consequently you deserve annihilation! That is why Scripture states: I am the Lord your God (Sifra, Acharei Mot, Chapter 13 22).