Muhammad ibn Ishaq reported from some of those knowledgeable in the earlier scriptures: Adam was intimate with Eve in Paradise before he fell into sin, and she conceived Cain and his twin sister Aqlima there. She experienced neither pregnancy cravings, illness, nor labor pains with them until she gave birth to them, and she saw no blood at their birth. Then, when Adam descended to the earth and was intimate with her, she conceived Abel and his twin sister. With them she experienced cravings, illness, labor pains, and blood.
Whenever Adam’s children reached maturity, he would marry a son from one pregnancy to a daughter from another. Thus, a man among them could marry whichever of his sisters he wished, except for the twin sister born with him, since at that time there were no women other than their sisters. When Cain and his twin sister Aqlima were born, followed by Abel and his twin sister Lubuda—and, according to al-Kalbi, there were two years between them—and they reached maturity, Allah Most High commanded Adam, peace be upon him, to marry Cain to Lubuda, Abel’s sister, and Abel to Aqlima, Cain’s sister.
Cain’s sister was more beautiful than Abel’s sister. Adam informed his children of this, and Abel was pleased, whereas Cain was displeased. He said, “She is my sister, and I have more right to her. We were born (1) in Paradise, whereas those two were born (2) on earth.” His father said to him, “She is not lawful for you.” But he refused to accept this and said, “Allah has not commanded him to do this; it is merely his own opinion.”
Adam, peace be upon him, said to them both, “Offer a sacrifice, and whichever of you has his sacrifice accepted shall have the greater right to her.” When sacrifices were accepted, a white fire would descend from heaven and consume them; but when they were not accepted, the fire would not descend, and birds and wild beasts would consume them. So they went out to offer a sacrifice (3).
Cain was a farmer, so he offered a heap of food from the poorest of his crop, secretly thinking to himself, “I do not care whether it is accepted from me or not; he shall never marry my sister.” Abel was a shepherd, so he selected the finest ram among his flock and offered it, inwardly seeking the pleasure of Allah, glorified and exalted is He. They placed their sacrifices upon the mount, and Adam, peace be upon him, supplicated. Then a fire descended from heaven and consumed Abel’s sacrifice, but did not consume Cain’s sacrifice (4). This is the meaning of His saying, “It was accepted from one of them”—that is, Abel (5)—“but it was not accepted from the other,” meaning Cain.
They then came down from the mountain, and Cain was angered that his sacrifice had been rejected. He harbored envy within himself until Adam went to Mecca to visit the House. When Adam was absent, Cain came to Abel while he was among his sheep and said, “I will surely kill you.” Abel asked, “Why?” Cain replied, “Because Allah Most High accepted your sacrifice and rejected mine, and you will marry my beautiful sister while I marry your ugly sister. Then people will say that you are better than I, and your children will boast over my children.” Abel said, “And what sin of mine is that? Allah accepts only from those who are mindful of Him.”