Midrash for Nehemiah 1:4

Cain spake before the Holy One, blessed be He: Sovereign of all the worlds ! "My sin is too great to be borne" (Gen. 4:13), for it has no atonement. This utterance was reckoned to him as repentance, as it is said, "And Cain said unto the Lord, My sin is too great to be borne" (ibid.); further, Cain said before the Holy One, blessed be He: Now will a certain righteous one arise on the earth and mention Thy great Name against me and slay me. What did the Holy One, blessed be He, do? He took one letter from the twenty-two letters, and put (it) upon Cain's arm that he should not be killed, as it is said, "And the Lord appointed a sign for Cain" (Gen. 4:15). The dog which was guarding Abel's flock also guarded his corpse from all the beasts of the field and all the fowl of the heavens. Adam and his helpmate were sitting and weeping and mourning for him, and they did not know what to do (with Abel), for they were unaccustomed to burial. A raven (came), one of its fellow birds was dead (at its side). (The raven) said: I will teach this man what to do. It took its fellow and dug in the earth, hid it and buried it before them. Adam said: Like this raven will I act. He took || the corpse of Abel and dug in the earth and buried it. The Holy One, blessed be He, gave a good reward to the ravens in this world. What reward did He give them? When they bear their young and see that they are white they fly from them, thinking that they are the offspring of a serpent, and the Holy One, blessed be He, gives them their sustenance without lack, as it is said, "Who provideth for the raven his food, when his young ones cry unto God, and wander for lack of meat" (Job 38:41). Moreover, that rain should be given upon the earth (for their sakes), and the Holy One, blessed be He, answers them, as it is said, "He giveth to the beast his food, and to the young ravens which cry" (Ps. 147:9).