If the Torah doesn’t tell us what was said, why do the rabbis present so many opinions. Redak on this verse, asks this question. What is his answer?
The Reason the Rabbis give different opinions is because the text isn’t clear, in this case the text doesn’t state what Cain told Hevel, therefore the Rabbis are explaining it.
The Radak’s answer is that after God told Cain that he should correct his ways, he got angry about it and blamed it all on Hevel. He also interprets the word EL (lit. to) to mean AL (lit. on or because of).