"And He will bless your bread" (Exodus 23:25). From where do we derive Grace after Meals from the Torah? etc. Rabbi Yitzhak says: It is not necessary, for behold it says, "And He will bless your bread"; do not read "u-verakh" [and He will bless] but "u-varekh" [and bless you], meaning: bless. And when is it called bread? Before one eats it.
Grace After Meals Derived from He Will Bless Your Bread
Yalkut Shimoni on Torah 359:9
(שמות כג כה) וּבֵרַךְ אֶת לַחְמְךָ. מִנַּיִן לְבִרְכַּת הַמָּזוֹן מִן הַתּוֹרָה וְכוּ'. רַבִּי יִצְחָק אוֹמֵר. אֵינוֹ צָרִיךְ, הֲרֵי הוּא אוֹמֵר וּבֵרַךְ אֶת לַחְמְךָ, אַל תִּקְרֵי 'וּבֵרַךְ' אֶלָּא 'וּבָרֵךְ', אֵימַת קָרוּי לֶחֶם, קֹדֶם שֶׁיֹּאכְלֶנּוּ.