The Two Sons of Abraham

Many rabbis and Muslim scholars believe Jews descend from Isaac and Arabs from Ishmael — making them cousins, as both Isaac and Ishmael are sons of Abraham عليه السلام,

Isaac عليه السلام,is the one who inherits prophethood from his father and the Divine covenant. 

Ishmael عليه السلام, is considered by many to be ethnically impure, as he was born from an Egyptian slave-girl named Hagar. Ishmael and his mother are eventually cast into the wilderness and he is depicted as a "wild man" (Genesis 16:12). The descendants of Isaac become the chosen people, while the descendants of Ishmael become a lost race steeped in ignorance and paganism.

But from the seed of Ishmael arises the final vindication of the rejected and the despised. From the unchosen people arises the Chosen One صلى الله عليه وسلم. The one who completes the chain of prophecy and honours both branches of Abrahams lineage. 

"Not one but two great nations were to look back at Abraham as their father."
    — Dr. Martin Lings

Notes

* Ishmael had grown to manhood by the time Abraham came to visit him in the wilderness. Together they set about building the House of God, the Ka’aba. When Abraham completed his task, never to return and never to see his son again, he blessed the valley of Mecca and prayed: ‘Our Lord, I have settled a part of my progeny in a barren valley close to Thy sacred House….’ (Quran 14:37); and he prayed also: ‘Our Lord, raise up in their midst a Messenger from amongst them, who shall recite to them Thy revelations and teach them the Scripture and Wisdom, and purify them’ (Quran 2:129)

The Bible has no record of this account since it is only concerned with the story of Isaac and his descendants. 

Writer Katheryn Pfisterer Darr questions whether the Biblical understanding of Ishmael and his mother (Hagar) may have been misjudged. God does not leave them to perish in the desert without leaving a trace. Darr cites recent Biblical scholarship that speaks of Hagar as an important person (the only woman in the Bible - along with the Virgin Mary - to be visited by an angel). Upon the request of Hagar, God not only saves Ishmael’s life in the desert, but promises that the descendants of Ishmael will - one day - make a great nation. In the words of Genesis:

“And God heard the voice of the lad; and the angel of God called to Hagar out of Heaven and said to her: what aileth thee, Hagar? Fear not, for God hath heard the voice of the lad where he is. Arise and lift up the lad and hold him in thy hand, for I will make him a great nation. And God opened her eyes and she saw a well of water.” - (21:17-19)