וַיֹּ֣אמֶר אֱלֹהִ֗ים אֲבָל֙ שָׂרָ֣ה אִשְׁתְּךָ֗ יֹלֶ֤דֶת לְךָ֙ בֵּ֔ן וְקָרָ֥אתָ אֶת־שְׁמ֖וֹ יִצְחָ֑ק וַהֲקִמֹתִ֨י אֶת־בְּרִיתִ֥י אִתּ֛וֹ לִבְרִ֥ית עוֹלָ֖ם לְזַרְע֥וֹ אַחֲרָֽיו׃
Lalu Allah berfirman: “Sesungguhnya, Sara, istrimu, akan melahirkan seorang anak laki-laki bagimu, dan engkau harus menamai dia Ishak. Aku akan meneguhkan perjanjian-Ku dengan dia sebagai perjanjian yang abadi bagi keturunannya sesudah dia.
Rashi
אבל VERILY — This word implies confirmation of a statement, just as (Genesis 42:21) “Verily (אבל) we are guilty”, and (2 Kings 4:14) “Verily (אבל) she has no son”.
Chizkuni
והקימותי את בריתי אתו, “I will maintain My covenant (the promise contained therein) with him.” G-d means that Yitzchok will be the first person to be circumcised on the eighth day of his life.
Ramban
‘AVAL’ SARAH THY WIFE. This is like the verse, “Aval” she has no son,457II Kings 4:14. the word aval meaning “only.” He thus said: “Only the son of whom I informed you, your wife Sarah will give birth to, and with him will I establish My covenant forever and with his seed after him; and Ishmael I will bless by making his children numerous, but this will not be because of My covenant with him.”