וַיֹּ֣אמֶר אֱלֹהִ֗ים אֲבָל֙ שָׂרָ֣ה אִשְׁתְּךָ֗ יֹלֶ֤דֶת לְךָ֙ בֵּ֔ן וְקָרָ֥אתָ אֶת־שְׁמ֖וֹ יִצְחָ֑ק וַהֲקִמֹתִ֨י אֶת־בְּרִיתִ֥י אִתּ֛וֹ לִבְרִ֥ית עוֹלָ֖ם לְזַרְע֥וֹ אַחֲרָֽיו׃
하나님이 가라사대 아니라 네 아내 사라가 정녕 네게 아들을 낳으리니 너는 그 이름을 이삭이라 하라 내가 그와 내 언약을 세우리니 그의 후손에게 영원한 언약이 되리라
Rashi on Genesis
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”.
Ramban on Genesis
‘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.”
Kli Yakar on Genesis
Sarah will bear you a son. In verse 16 God did not mention that Sarah would actually bear a son, so Avraham thought that perhaps she would adopt one.