Tosefta zu Schemot 24:12

The Beracha (blessing) of Zimun is from the Torah (i.e. a Torah obligation) as it is said, “And you should eat, and be satisfied, and bless…” (Deuteronomy 8:10), this is [a reference to] the Beracha of Zimun. “… Hashem, your God …” (Deuteronomy 8:10), this is [a reference to] the first Beracha [of Birkat Hamazon (Grace After Meals)]. “… for the land …” (Deuteronomy 8:10), this is [a reference to] the Beracha about the land (i.e. the second Beracha) [of Birkat Hamazon]. “… the good …” (Deuteronomy 8:10), this is [a reference to the Beracha about] Jerusalem (i.e. the third Beracha) [of Birkat Hamazon] as it is said “… this good mountain and Levanon.” (Deuteronomy 3:25). “… which He has given you.” (Deuteronomy 8:10), this is [a reference to the Beracha of] Hatov Vehameitiv (The Good and Who does good) (i.e. the fourth Beracha) [of Birkat Hamazon].

From where [do we know] that just like you have to say a Beracha (blessing) after [eating food] you also have to say a Beracha before [eating food]? The Torah teaches us, “… which He has given you” (Deuteronomy 8:10), [meaning] from the moment that He gave [the food] to you, [you have to say a Beracha on it.] From where [do we know] that [you have to say a Beracha] even on [seeing] mountains and hills? The Torah teaches us, “… for the land …” (Deuteronomy 8:10). From where [do we know] that [you have to say a Beracha] even on [studying and reading] the Torah and on [performing] Mitzvot (commandments)? The Torah teaches us, “… which He has given you” (Deuteronomy 8:10), and it says, “… and I will give you the tablets of stone …” (Exodus 24:12).