After Cain’s dialogue with God, Cain met with Adam.
Said Adam to him: “What was the conclusion of your judgment?”
Said Cain: “I repented and was reconciled.”
Whereupon Adam slapped himself in the face, saying: Such is the power of teshuva, and I did not know it! It was then that Adam recited מִזְמוֹר שִׁיר, לְיוֹם הַשַּׁבָּת. (Tehillim 92) ~ Midrash Rabbah