Ramban on Genesis 26:3:1

וַהֲקִמֹתִי אֶת הַשְּׁבוּעָה אֵין צָרִיךְ שֶׁיַּבְטִיחַ הקב"ה אֶת יִצְחָק שֶׁלֹּא יַעֲבֹר הַשְּׁבוּעָה שֶׁנִּשְׁבַּע לְאָבִיו, כִּי לֹא אָדָם הוּא לְהִנָּחֵם (שמואל א טו כט), וְאַבְרָהָם אֵין לוֹ זֶרַע אַחֵר בַּעַל בְּרִית לֵאלֹהִים, וְהַשְּׁבוּעָה אֵינֶנָּה עַל תְּנַאי, כִּי בְּיַעֲקֹב הֻצְרַךְ מִפְּנֵי עֵשָׂו אָחִיו לוֹמַר שֶׁבּוֹ יִתְקַיֵּם הַבְּרִית וּבְזַרְעוֹ (בראשית ל"ה:י"ב), לֹא בְּעֵשָׂו. וְנִרְאֶה שֶׁזֶּה הַמַּאֲמָר "וַהֲקִמֹתִי אֶת הַשְּׁבוּעָה" יֵחָשֵׁב שְׁבוּעָה, וּלְכָךְ תָּמִיד יֹאמַר בַּתּוֹרָה (שמות לג א) "הָאָרֶץ אֲשֶׁר נִשְׁבַּעְתִּי לְאַבְרָהָם לְיִצְחָק וּלְיַעֲקֹב", "אֲשֶׁר נִשְׁבַּעְתָּ לָהֶם בָּךְ" (שם לב יג), כִּי לֹא מָצָאנוּ שְׁבוּעָה לְיִצְחָק בִּלְתִּי זֹאת. וְרָצָה הקב"ה לְהִשָּׁבַע לְכָל אֶחָד מִן הָאָבוֹת, לְהוֹדִיעַ שֶׁרָאוּי כָּל אֶחָד לִכְרֹת עִמּוֹ בְּרִית, וְשֶׁתִּהְיֶה זְכוּת כָּל אֶחָד עוֹמֶדֶת לְפָנָיו עִם זַרְעָם, כִּי אַף עַל פִּי שֶׁהָרִאשׁוֹנָה תַּסְפִּיק, תּוֹסֶפֶת זְכוּת וְכָבוֹד הוּא לָהֶם. וְלָכֵן אָמַר (ויקרא כו מב) "וְזָכַרְתִּי אֶת בְּרִיתִי יַעֲקֹב וְאַף אֶת בְּרִיתִי יִצְחָק וְאַף אֶת בְּרִיתִי אַבְרָהָם אֶזְכֹּר וְהָאָרֶץ אֶזְכֹּר", כִּי כֻּלָּם בַּעֲלֵי בְּרִית לֵאלֹהִים. וְיִתָּכֵן שֶׁהוֹסִיף לוֹ לְיִצְחָק בַּשְּׁבוּעָה הַזֹּאת שֶׁיְּקַיֵּם בּוֹ בְּעַצְמוֹ הַשְּׁבוּעָה אֲשֶׁר נִשְׁבַּע לְאַבְרָהָם אָבִיו, שֶׁיִּהְיֶה הוּא בְּרָכָה בָּעַמִּים, כַּאֲשֶׁר אָמַר לְאַבְרָהָם אָבִיו "וְהִתְבָּרֲכוּ בְזַרְעֲךָ כֹּל גּוֹיֵי הָאָרֶץ", וְיִהְיֶה פֵּרוּשׁ הַכָּתוּב וַהֲקִמֹתִי בְּךָ אֶת הַשְּׁבֻעָה אֲשֶׁר נִשְׁבַּעְתִּי לְאַבְרָהָם אָבִיךָ, כִּי אַתָּה בְּרָכָה בָּעַמִּים. וְכֵן אָמַר עוֹד בְּיַעֲקֹב (בראשית כ"ח:י"ד) "וְנִבְרְכוּ בְךָ כָּל מִשְׁפְּחֹת הָאֲדָמָה וּבְזַרְעֶךָ": AND I WILL FULFILL THE OATH WHICH I SWORE UNTO ABRAHAM THY FATHER. There is no need for the Holy One, blessed be He, to assure Isaac that He will not violate the oath which He swore to his father, for He is not a man, that He should repent.¹²⁷I Samuel 15:29. Abraham had no other seed upon whom a covenant had been established with G-d except Isaac. The oath, moreover, was not given on condition. In the case of Jacob,¹²⁸Further, 35:12. And [[illegible]] land which I gave to Abraham and Isaac, to thee will I give it. it was necessary that he be given such assurance on account of his brother Esau. He was thus saying that in him [Jacob] and his seed will the covenant be fulfilled, not in Esau. [But in the case of Isaac, why was it necessary that he be given such a promise?]
It would appear then that this expression, Vehakimothi eth hashevuah, is itself an oath.¹²⁹It is thus to be translated, And I will ‘establish’ the oath. This is now found in most English translations. It is for this reason that the Torah always says, The land which I swore unto Abraham, unto Isaac, and unto Jacob;¹³⁰Deuteronomy 34:4. Remember Abraham, Isaac, and Israel, Thy servants, to whom Thou didst swear by Thine own self.¹³¹Exodus 32:13. For we find no source for an oath having been given to Isaac except this verse.
Now it was the desire of the Holy One, blessed be He, to swear to each one of the patriarchs to let it be known that each one was worthy of the covenant being made with him alone, and that the merit of each one stands before Him together with their seed. Even though the previous one suffices, it is an additional merit and honor to them.¹³²That is, their descendants. It is for this reason He said, Then will I remember My covenant with Jacob, and also My covenant with Isaac, and also My covenant with Abraham will I remember; and I will remember the land,¹³³Leviticus 26:42. since all of them had the distinction of G-d having made a covenant with them.
It is possible that He promised something additional to Isaac through this oath, namely, that He will fulfill in him himself, the oath He had sworn to Abraham his father, i.e., that he [Isaac] will be a blessing among the nations, even as He said to Abraham his father, And all the nations of the earth shall bless themselves with thy seed.¹³⁴Above, 22:18. The explanation of the verse before us will thus be: “And I will fulfill in thee the oath which I swore unto Abraham thy father since you will be a blessing among the nations.” Similarly, He also says in the case of Jacob, And in thee and in thy seed shall all of the families of the earth be blessed.¹³⁵Further, 28:14.