Rashi on Genesis 26:4/רש"י על בראשית כ״ו:ד׳

1והתברכו בזרעך AND IN THY SEED SHALL BLESS THEMSELVES — A man will say to his son, “May your seed be as the seed of Isaac”. Such is the meaning of this phrase wherever it occurs in the entire Scriptures. The following passage is that from which this meaning may be derived for all such passages: (48:20) “By thee shall all Israel bless their children saying, “May God make thee [as Ephraim and Manasseh]”. So, too, in the case of a curse do we find a similar idea: (Numbers 5:27) “And the woman shall become a curse”, meaning that one who curses his enemy will say “May you be like such and such a woman”. Similar, also, is (Isaiah 65:15) “And ye shall leave your name for a curse unto mine elect”, meaning that one who takes an oath will say “May I be like such-and-such a person if I have really done so-and-so”.