(Exodus 1:6:) "And Joseph died, and all his brothers." Even though they had died, (Exodus 1:7:) "and the children of Israel were fruitful and swarmed." Rabbi Yochanan said: Every woman of Israel would give birth to sixty. As it is stated (in the same verse): "and the land was filled with them." Rabbi Yochanan said: They filled Egypt.
Joseph Died Yet Israel Multiplied and Filled All of Egypt
Midrash Tanchuma Buber, Shemot 6:1
וימת יוסף וכל אחיו (שמות א ו). אע"פ שמתו, ובני ישראל פרו וישרצו (שם שם ז), אמר ר' יוחנן, כל אחת מישראל היתה יולדת ששים, שנאמר ותמלא הארץ אתם (שם), אמר ר' יוחנן מילאו את מצרים.