פירוש על בראשית 17:19

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”.

‘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.”

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.