"…Your name will no longer be Abram, but Abraham." (Gen. 17:5)

The Midrash already stated that when Abram cited that he had seen in his horoscope that he would not father any children, G‑d agreed with him and told him that he was correct, since as "Abram" he would not father children but as "Abraham" he would. The same would apply to "Sarai", as opposed to "Sarah". (Bereishit Rabba 44:10)

Whatever he had foreseen in his horoscope applied to himself in conjunction with his wife….

This Midrash is very difficult, since the Torah reports Abram as siring Ishmael, not Abraham! Besides, why did the Midrash mention that Sarai did not bear children when Abraham had never even raised the problem of Sarai not having children? We must conclude therefore that Abraham already knew at that time or at least surmised that any meaningful seed of his could only be conceived through Sarah because (presumably) whatever he had foreseen in his horoscope applied to himself in conjunction with his wife. G‑d confirmed this when He told him that his seed would be known through Isaac. This is why the Midrash included the name Sarah when describing a prophecy primarily directed at Abraham. Abraham realized that the need for Ishmael to be his son was to rid his sperm of any residual contamination of the original serpent. Fathering Ishmael, then, was not "fathering" in the true sense of the word.

"G‑d said: 'But your wife Sarah will bear a son for you.'" (Gen. 17:19)

Why did G‑d add the word "but"? G‑d may have wanted to explain to Abraham that if the gift of a son was something intended only for Abraham, it would suffice to grant his prayer and assist Ishmael to become worthy. However, there was another person to be considered, his wife Sarah. Inasmuch as Sarah was destined to bear a son, he, Abraham, had no right to waive the gift of a son by Sarah in order for Ishmael to grow up worthy of his father.

The Torah was very particular using the word "lecha", meaning "for you", in connection with the son Sarah would bear. She would bear this son in answer to her prayer.

It is also possible that the Torah here hinted at G‑d's displeasure that Abraham was satisfied with Ishmael, someone who contained residual poisonous material from the original serpent. Had it not been for Abraham's prayer (in the previous verse) regarding the future of Ishmael, his son Isaac might not have originated in what kabbalists know as the "Sitra Denukva", an emanation on the left and female side of the sefirot. Had Isaac originated in the other side of the sefirot, he might never have fathered an Esau. The matter had ramifications at the time of the Binding of Isaac, when Isaac received the soul who would bear children for him, i.e. the moment at which Rebecca, his life-partner, was born.

[Selected with permission from the five-volume English edition of "Ohr HaChaim: the Torah Commentary of Rabbi Chaim Ben Attar" by Eliyahu Munk.]