רש”י על דברים: א:ז

עד הנהר הגדל.

מִפְּנֵי שֶׁנִּזְכָּר עִם אֶרֶץ יִשְׂרָאֵל קוֹרְאוֹ גָּדוֹל, מְשַׁל הֶדְיוֹט אוֹמֵר “עֶבֶד מֶלֶךְ מֶלֶךְ”, “הִדַּבֵּק לַשַּׁחֲוָר וְיִשְׁתַּחֲווּ לְךָ”, ״קְרַב לְגַבֵּי דְהִינָא וְאִדַּהֵן” (שבועות מ”ז): עד הנהר הגדל TO THE GREAT RIVER [THE RIVER EUPHRATES]
— Because it is mentioned in connection with the Land of Israel, it terms it “great”. A popular proverb says: A king’s servant is a king, attach yourself to a captain and people will bow down to you; go near to an anointed (a distinguished) person and you become anointed (distinguished) yourself (cf. Rashi on Shevuot 47b and Genesis 15:18).