The First Murder - from Various Hadith

Ibn Abbas, Ibn Masud and a group of the companions of the Prophet related that inter-marriage of the male of one pregnancy with the female of another had been in practice among Adam's children. Abel wanted to marry Cain's sister, but Cain wanted her for himself because she was very beautiful. Adam ordered him to give her in marriage to his brother, but he refused. So Adam ordered both of them to offer a sacrifice, then went to Mecca (Makkah) to perform the pilgrimage. After Adam had left, they offered their sacrifices: Abel offered a fat lamb - he was a shepherd - while Cain offered a bundle of the worst grain. A fire descended and devoured Abel's sacrifice, leaving that of Cain, so he became angry and said: "I will surely kill you so that you will not marry my sister." Abel replied: "Allah accepts from those who fear Him."

According to Abu Jaafar Al-Baqer, Adam was watching their offering and was sure that Abel's sacrifice would be accepted. Cain complained to Adam that the acceptance was due to his supplication for Abel and that he had not done the same for him, so he promised his father to settle the matter between himself and his brother. One night, Abel was late returning from tending his flock. Adam sent Cain to see what had happened to him. When he found him, he glared at him saying: "Yours was accepted, and mine was not." Abel replied: "Allah only accepts from the Allah-fearing." Cain became angry on hearing this and hit him with a piece of iron that was with him, thus killing him. In another version it was said that he killed him with a rock to the head while he was sleeping.