TT #98 · Parshas Shemini · Aliyah 6 · Passuk 4
Chabad.org — Vayikra Chapter 11 with Rashi
English: “Only this you shall not eat”: because both the positive permission and negative prohibition are stated, each supplies a separate command, creating both a positive and a negative prohibition.
Chullin 66b.
E-Daf.com — Chullin 66b (Tsurras Hadaf)
אַךְ אֶת זֶה לֹא תֹאכֵלוּ. תָּנוּ רַבָּנַן, מִמַּשְׁמַע שֶׁנֶּאֱמַר אֱכֹל אֶת שֶׁיֵּשׁ לוֹ, שׁוֹמֵעַ אֲנִי אַל תֹּאכַל אֶת שֶׁאֵין לוֹ, וּמִמַּשְׁמַע שֶׁנֶּאֱמַר אַל תֹּאכַל אֶת שֶׁאֵין לוֹ שׁוֹמֵעַ אֲנִי אֱכֹל אֶת שֶׁיֵּשׁ לוֹ, וְלָמָּה שְׁנָאָן לְכָל אֶחָד – לַעֲבֹר עָלָיו בַּעֲשֵׂה וְלֹא תַעֲשֶׂה.
Textual analysis — The Chazal derives the teaching from the wording or classification in the verse.
The two reciprocal formulations teach both the positive and negative dimensions: eating what has the required sign is commanded, while eating what lacks it is prohibited.