(Devarim 17:16) "Only he shall not multiply for himself horses": I might think not (even enough) for his chariot and his charioteers; it is, therefore, written "he shall not multiply for himself" — but he may multiply for his chariot and his charioteers.
Now does this not follow a fortiori, viz.: If a mitzvah on which the covenant hinges (i.e., to go to Eretz Yisrael and not to return to Egypt [viz. Shemoth 14:12]) — a transgression (in this instance, returning there for horses,) returns them to Egypt, (this, the thrust of "that he not return the people to Egypt in order to add a horse"), then (returning them to the) other lands, on which the covenant does not hinge, how much more so (will a transgression return them there)!
(דברים יז טז) רַק לֹא יַרְבֶּה לּוֹ סוּסִים, יָכוֹל לֹא יַרְבֶּה לְמֶרְכַּבְתּוֹ וּלְפָרָשָׁיו? תַּלְמוּד לוֹמַר: לוֹ, לוֹ אֵינוֹ מַרְבֶּה אֲבָל מַרְבֶּה הוּא לְמֶרְכַּבְתּוֹ וּלְפָרָשָׁיו.
(Devarim 17:16) "Only he shall not multiply for himself horses": I might think not (even enough) for his chariot and his charioteers; it is, therefore, written "he shall not multiply for himself" — but he may multiply for his chariot and his charioteers.
אִם כֵּן לָמָּה נֶאֱמַר לְמַעַן הַרְבּוֹת סוּס? סוּסִים בְּטֵלִים.
How, then, is "horses" to be understood? As "idle" horses.
מִנַּיִן אֲפִלּוּ סוּס אֶחָד וְהוּא בָּטֵל, כְּדַי הוּא שֶׁיַּחֲזִיר אֶת הָעָם לְמִצְרַיִם? תַּלְמוּד לוֹמַר: רַק לֹא יַרְבֶּה לּוֹ סוּסִים וְלֹא יָשׁוּב
Whence is it derived that the addition of even one idle horse constitutes a transgression? From (Ibid.) "in order to multiply a horse."
וַהֲלֹא דְּבָרִים קַל וָחֹמֶר: וּמָה מִצְרַיִם, שֶׁהַבְּרִית כְּרוּתָה עָלֶיהָ, עֲבֵרָה מַחֲזִירָתָהּ לְשָׁם, שְׁאָר אֲרָצוֹת, שֶׁאֵין הַבְּרִית כְּרוּתָה עֲלֵיהֶן – עַל אַחַת כַּמָּה וְכַמָּה.
Now does this not follow a fortiori, viz.: If a mitzvah on which the covenant hinges (i.e., to go to Eretz Yisrael and not to return to Egypt [viz. Shemoth 14:12]) — a transgression (in this instance, returning there for horses,) returns them to Egypt, (this, the thrust of "that he not return the people to Egypt in order to add a horse"), then (returning them to the) other lands, on which the covenant does not hinge, how much more so (will a transgression return them there)!