The Quran does not confine prophethood to Isaac's line. Quran 29:27 speaks about Abraham's descendants, not an Isaac-only monopoly; the Quran explicitly calls Ishmael "a messenger and a prophet"; and the authentic Sunnah identifies Muhammad (PBUH) as from the children of Ishmael.
What Quran 29:27 Actually Says
And We granted him Isaac and Jacob and placed in his descendants prophethood and scripture, and We gave him his reward in this world, and indeed, in the Hereafter he will be among the righteous.
Quran 29:27
The verse is about Abraham. He is the central figure of the passage, and the repeated pronoun in "We granted him" and "his descendants" naturally returns to him. Classical tafsir treats the phrase this way: al-Tabari explains "his descendants" as the descendants of Abraham, and Ibn Kathir connects this verse with Quran 57:26, explaining that prophethood and revealed scripture were placed among Abraham's offspring.
The verse does mention Isaac and Jacob because that branch of Abraham's family was given immense honor, and many prophets and scriptures came through the Children of Israel. But honoring Isaac and Jacob is not the same as excluding Ishmael. The verse does not say "in Isaac's descendants alone" or "only through Jacob." There is no wording of exclusivity in the verse.
The Broader Quranic Pattern
Another verse states the matter in a broader Abrahamic context:
And We have already sent Noah and Abraham and placed in their descendants prophethood and scripture; and among them is he who is guided, but many of them are defiantly disobedient.
Quran 57:26
This is a statement that Allah placed prophethood and scripture among the offspring of Noah and Abraham. It is not an Isaac-only rule. Ishmael is Abraham's son, and Muhammad (PBUH) being from Ishmael's descendants still falls within Abraham's blessed progeny.
Ishmael Is Explicitly a Prophet
And mention in the Book, Ishmael. Indeed, he was true to his promise, and he was a messenger and a prophet.
Quran 19:54
This verse is decisive. The Quran does not merely describe Ishmael as righteous; it calls him both a messenger and a prophet. Therefore, the claim that prophethood is confined to Isaac's line is directly contradicted by the Quran itself.
The Quran also records Abraham and Ishmael, while raising the foundations of the Kaaba, praying for a messenger among their descendants. Immediately before this prayer, they ask Allah to make "from our descendants a Muslim nation" (Quran 2:128). Then they say:
Our Lord, and send among them a messenger from themselves who will recite to them Your verses and teach them the Book and wisdom and purify them. Indeed, You are the Exalted in Might, the Wise.
Quran 2:129
In context, "among them" refers to the descendants being prayed for by Abraham and Ishmael in connection with the House in Makkah. This is not an Isaac-only horizon.
Muhammad (PBUH) and the Children of Ishmael
The authentic Sunnah explicitly connects the Prophet Muhammad (PBUH) to the children of Ishmael:
Indeed, Allah chose Kinana from the children of Ishmael, chose Quraysh from Kinana, chose Banu Hashim from Quraysh, and chose me from Banu Hashim.
Sahih Muslim
The Quran identifies Muhammad (PBUH) as the final prophet:
Muhammad is not the father of any one of your men, but he is the Messenger of Allah and Seal of the Prophets. And ever is Allah, of all things, Knowing.
Quran 33:40
And his mission is universal, not tribal or local:
And We have not sent you except to all mankind, as a bringer of good tidings and a warner. But most of the people do not know.
Quran 34:28
Conclusion
The objection fails because it reads restriction into a verse that contains no restriction. Quran 29:27 honors Abraham's descendants; it does not confine prophethood to Isaac's descendants alone. Ishmael is explicitly named as a messenger and prophet, Abraham and Ishmael pray for a messenger among their descendants, and the authentic Sunnah identifies Muhammad (PBUH) as from the children of Ishmael. The final prophet arising from Ishmael's line is therefore fully consistent with the Quranic account of Abrahamic prophethood.