מדרש אויף בראשית 4:9

And the Lord knew what Cain had done unto his brother and the Lord appeared unto Cain and ‎said unto him: Where is thy brother Abel who was with thee? But Cain denied all knowledge of ‎Abel and said: I know not; am I my brother's keeper? And the Lord said unto him: What hast ‎thou done? The voice of thy brother's blood crieth unto me from the ground; thou hast killed ‎thy brother and hast denied it to me, thinking in thine heart that I have not seen thee, and ‎that I would not know all the deeds that thou hast done.‎ And now that thou hast killed thy brother, simply because he told unto thee the truth, be thou ‎therefore cursed from the earth which hath opened her mouth to receive the blood of thy ‎brother from thy hand, and because thou hast buried him in her.‎ And when thou tillest the ground it shall not henceforth yield unto thee its strength as ‎heretofore, but it shall bring forth unto thee thorns and thistles also. A fugitive and a vagabond ‎shalt thou be in the earth until the day of thy death.‎ And Cain went out from the presence of the Lord from the place that was his home, and he ‎wandered around a fugitive and a vagabond on the earth eastward of Eden, he and all that ‎were his.‎ And Cain knew his wife at that time and she conceived and bare a son, and he called his name ‎Enoch, saying: In those days the Lord began to give me peace and rest in the land. At the same ‎time Cain began building a city and when the city was built he called her name Enoch, after the ‎name of his son; for the Lord had given him rest upon the earth in those days, and he did not ‎live any more like a fugitive and a vagabond as theretofore.‎ And unto Enoch was born Irad; and Irad begat Mehujael; and Mehujael begat Methusael; and ‎Methusael begat Lamech.‎ And it was in the one hundred and thirtieth year of the life of Adam upon the earth, that Adam ‎knew Eve his wife and she conceived and bare a son in his image and likeness, and called his ‎name Seth saying: God hath appointed unto me another offspring instead of Abel whom Cain ‎hath killed.‎ And Seth lived one hundred and five years and he begat a on.‎ And Seth called the name of his son Enos, saying: At that time the sons of man began to ‎multiply upon the face of the earth and to injure their hearts and souls by rebelling and ‎transgressing against the Lord.‎ And it was in the days of Enos that the sons of man continued the more so to rebel and ‎transgress against the Lord, and to increase the burning anger of the Lord against the sons of ‎man. For the sons of man went and served other gods, and forgot the Lord who created them ‎upon the earth.‎ And the sons of man of those days, made unto themselves images of iron and of copper, and ‎of wood and of stone, and bowed down before them, and served them.‎ And every man made his own god and bowed down before it, and the sons of man forsook ‎the Lord through all the days of Enos and his sons.‎

After killing Abel, he said to himself: “I must flee from my mother and father, for they will ask me where he is, since there are only the two of us.” The Holy One, blessed be He, appeared before him immediately, saying: “You can flee from your parents but not from Me, as it is said: Can any man hide himself in secret places that I shall not see him? saith the Lord. Do I not fill heaven and earth? (Jer. 23:24).” The Holy One asked: Where is Abel, your brother? (Gen. 4:9), and then He continued: “Woe unto him who was merciful to you and refrained from killing you when you were beneath him. When he permitted you to rise, you killed him!”

How did he kill him? He inflicted numerous bruises upon his body with a stone. He smote him over his entire body, from his hands and feet to his throat, for he had no way of knowing from where his soul would depart. When the Holy One, blessed be He, asked him: Where is Abel, your brother? he replied: “I know not; am I my brother’s keeper? (Gen. 4:9). You are the guardian of all, yet You seek him from me!” This may be compared to a thief who steals during the night but is not apprehended while committing the crime. When the watchman seizes him the next morning and asks: “Why did you steal those utensils?”, the thief replies: “I am a thief and I did not neglect my profession. Your job is to keep watch at the gate. Why did you neglect your duties? Now you question me?” So Cain retorted: “True, I slew him, but You created the evil inclination within me. Since You are the guardian of all, why did You permit me to slay him? You who are called the I³³I am the Lord your God (Exod. 20:2). killed him, for if You had accepted my sacrifice as You accepted his, I would not have been envious of him.” The Holy One, blessed be He, replied immediately: What have you done? The voice of thy brother’s blood crieth unto Me (ibid., v. 10). This verse indicates that he inflected numerous wounds upon Abel.