Midrash Tanchuma, Bereshit 10

‎הֵן גֵּרַשְׁתָּ אֹתִי וְגוֹ', וַיֹּאמֶר לוֹ ה' לָכֵן כָּל הֹרֵג קַיִן וְגוֹ'. יֵשׁ אוֹמְרִים: שַׁבָּת נָעַל בְּפָנָיו, כְּמוֹ שֶׁנֶּאֱמַר: בֵּינֵי וּבֵין בְּנֵי יִשְׂרָאֵל אוֹת הִיא לְעֹלָם (שמות לא, יז). כְּשֵׁם שֶׁלִּמֵּד שַׁבָּת זְכוּת עַל אָדָם הָרִאשׁוֹן, כָּךְ לִמֵּד עַל קַיִן. וְיֵשׁ אוֹמְרִים: קֶרֶן קָבַע בְּמִצְחוֹ.

Behold, Thou hast driven me out this day from the face of the land … and the Lord said unto him: “Therefore, whosoever slayeth Cain, vengeance shall be taken on him sevenfold.” and the Lord set a sign for Cain (Gen. 4:14–15). There are some who say that the word “Sabbath” was placed as a sign upon his countenance, as it is said: My Sabbaths, for it is the sign between Me and you, throughout your generations (Exod. 31:13), and that just as the Sabbath pleaded in behalf of Adam,35The Sabbath pleaded with God to spare Adam’s life lest the glory and the sanctity of the Sabbath be impaired by his death. it pleaded in behalf of Cain. Others, however, insist that He fastened a horn upon his forehead.36Bereshit Rabbah lists seven signs placed on Cain.

בְּשָׁעָה שֶׁהָרַג קַיִן אֶת הֶבֶל הָיָה מֻשְׁלָךְ וְלֹא הָיָה יוֹדֵעַ קַיִן מַה לַּעֲשׂוֹת. זִמֵּן לוֹ הַקָּדוֹשׁ בָּרוּךְ הוּא שְׁנֵי עוֹפוֹת טְהוֹרִים וְהָרַג אֶחָד מֵהֶן אֶת חֲבֵרוֹ וְחָפַר בְּיָדָיו וּקְבָרוֹ, וּמִמֶּנּוּ לָמַד קַיִן וְחָפַר וְקָבַר אֶת הֶבֶל. לְפִיכָךְ זָכוּ הָעוֹפוֹת לְכַסּוֹת אֶת דָּמָן.

After Cain slew Abel, the body lay outstretched upon the earth, since Cain did not know how to dispose of it. Thereupon, the Holy One, blessed be He, selected two clean birds and caused one of them to kill the other. The surviving bird dug the earth with its talons and buried its victim. Cain learned from this what to do. He dug a grave and buried Abel. It is because of this that birds are privileged to cover their blood.37A raven taught Adam how to bury Abel. Ginzberg, Legends of the Jews 5:142; 1:113.