Ibn Ezra on Genesis 4:8/אבן עזרא על בראשית ד׳:ח׳

1AND CAIN SPOKE. It appears to me that Cain related to Abel the full account of the rebuke with which God had reproached him. Those born on a dark day ask, how did Cain kill Abel since no swords were yet in existence? This is a foolish question. He could have choked him or killed him with a stone or a piece of wood inasmuch as there were thousands of stones and chunks of wood around.