Rashbam on Exodus
1:1א׳:א׳
1 א

ואלה שמות - מפני שרוצה לפרש ולומר ובני ישראל פרו וישרצו וגו', הוצרך לכפול ולומר בבואם למצרים לא היו אלא שבעים ואחר מות דור ההוא פרו וישרצו.

ואלה שמות, seeing that the Torah wanted to let us know how the Israelites had increased and multiplied (verse 3) it became necessary to repeat that when they had arrived in Egypt they had numbered only 70 souls. The dramatic increase in numbers of Israelites began only with the death of the generation that had moved there from the land of Canaan. As a result, when a new king came to the throne in Egypt, he wanted to diminish their numbers and did not succeed in doing so.