Rashi on Genesis 5:1/רש"י על בראשית ה׳:א׳

1זה ספר תולדות אדם THIS IS THE BOOK OF THE GENERATIONS OF ADAM — This is the record (or, enumeration) of the generations of Adam. There are however, many Midrashic observations on this verse

2‘ביום ברא וגו IN THE DAY THAT [GOD] CREATED etc. — This states that on the day when he was created he begot progeny (Genesis Rabbah 24:7).