Chapter. 77Original Source http://www.sacred-texts.com/bib/fbe/fbe082.htm
(i) Cain, 15 years old, and Abel 12 years old, grow apart.
1 BUT when Adam saw that the elder brother hated the younger, he endeavoured to soften their hearts, and said unto Cain, “Take, O my son, of the fruits of thy sowing, and make an offering unto God, that He may forgive thee thy wickedness and thy sin.”
2 He said also to Abel, “Take thou of thy sowing and make an offering and bring it to God, that He may forgive thy wickedness and thy sin.”
3 Then Abel hearkened unto his father’s voice, and took of his sowing, and made a good offering, and said to his father, Adam, “Come with me, to show me how to offer it up.”
4 And they went, Adam and Eve with him, and showed him how to offer up his gift upon the altar. Then after that, they stood up and prayed that God would accept Abel’s offering.
5 Then God looked upon Abel and accepted his offering. And p. 56 God was more pleased with Abel than with his offering, because of his good heart and pure body. There was no trace of guile in him.
6 Then they came down from the altar, and went to the cave in which they dwelt. But Abel, by reason of his joy at having made his offering, repeated it three times a week, after the example of his father Adam.
7 But as to Cain, he took no pleasure in offering; but after much anger on his father’s part, he offered up his gift once; and when he did offer up, his eye was on the offering he made, and he took the smallest of his sheep for an offering, and his eye was again on it. ‘
8 Therefore God did not accept his offering, because his heart was full of murderous thoughts.
9 And they all thus lived together in the cave in which Eve had brought forth, until Cain was fifteen years old, and Abel twelve years old.
Original Source: The First Book of Adam and Eve Also called: The Conflict of Adam and Eve with Satan. http://www.sacred-texts.com/bib/fbe/index.htm#section_000