Vayeshev: The Shevatim Keeping the Commandments

There is a good proof in parshat Vayeshev that the Avot, and Shevatim, kept the Torah before it was given, and I don't think I've remarked on it before, so here it is before I forget:

Bereishit 37:26:

כד וַיִּקָּחֻהוּ--וַיַּשְׁלִכוּ אֹתוֹ, הַבֹּרָה; וְהַבּוֹר רֵק, אֵין בּוֹ מָיִם.	24 and they took him, and cast him into the pit--and the pit was empty, there was no water in it.
כה וַיֵּשְׁבוּ, לֶאֱכָל-לֶחֶם, וַיִּשְׂאוּ עֵינֵיהֶם וַיִּרְאוּ, וְהִנֵּה אֹרְחַת יִשְׁמְעֵאלִים בָּאָה מִגִּלְעָד; וּגְמַלֵּיהֶם נֹשְׂאִים, נְכֹאת וּצְרִי וָלֹט--הוֹלְכִים, לְהוֹרִיד מִצְרָיְמָה.	25 And they sat down to eat bread; and they lifted up their eyes and looked, and, behold, a caravan of Ishmaelites came from Gilead, with their camels bearing spicery and balm and ladanum, going to carry it down to Egypt.
כו וַיֹּאמֶר יְהוּדָה, אֶל-אֶחָיו: מַה-בֶּצַע, כִּי נַהֲרֹג אֶת-אָחִינוּ, וְכִסִּינוּ, אֶת-דָּמוֹ.	26 And Judah said unto his brethren: 'What profit is it if we slay our brother and conceal his blood?

The implication is that if they would have slaughtered him, then certainly they would have performed kisui hadam.

QED.