When Adam had lived 130 years, he fathered a son in his own likeness, according to his image, and named him Seth.
Genesis 5:3 (NASB)
Genesis 5 says his image about Seth, and nobody else.
The Rabbis wondered about this and suggest that Adam and Eve may have had other offspring after Cain and Abel, before Seth. These offspring were… different. Not like Adam. Something monstrous.
This points to the Nephilim in Genesis 6.