Reference for Chullin 7:6
נוֹהֵג בִּטְהוֹרָה, וְאֵינוֹ נוֹהֵג בִּטְמֵאָה. רַבִּי יְהוּדָה אוֹמֵר, אַף בִּטְמֵאָה. אָמַר רַבִּי יְהוּדָה, וַהֲלֹא מִבְּנֵי יַעֲקֹב נֶאֱסַר גִּיד הַנָּשֶׁה, וַעֲדַיִן בְּהֵמָה טְמֵאָה מֻתֶּרֶת לָהֶן. אָמְרוּ לוֹ, בְּסִינַי נֶאֱמַר, אֶלָּא שֶׁנִּכְתַּב בִּמְקוֹמוֹ:
The prohibition of the גיד הנשה [sciatic nerve] applies to clean animals, and not to unclean ones. R. Judah said, "It must be observed also in respect to unclean animals;" for he argued thus, "The גיד הנשה [sciatic nerve] was prohibited since the time of the sons of Jacob [i.e. before the promulgation of the law], when it was not yet prohibited to use unclean animals as food." The sages replied, "This precept was first promulgated at Sinai, but it was written [incidentally] in its place."