The Quran at Surah 29, Aya 27, admits that prophet Mohammad (PBHU) could not be a prophet since it stipulates that prophets will come in the line of Abraham, Isaac, and Jacob. Mohammad (PBUH), according to Muslims, came in the line of Ishmael, which means he could not be a prophet and Mohammad was not related to Ishmael.

Also, Genesis 17:21-22, says God made a covenant with Isaac, NOT Ishmael. Deuteronomy 18:18 states that only a "brethren" of the Israelites, as in a fellow Israelite, could be the prophet.