First Abraham married Hagar as it says in the TORAH Genesis 16:3, "After 
Abram had lived in Canaan for ten years, his wife Sarai took Hagar the Egyptian 
her slave, and gave her to her husband Abram as a WIFE."  So thusly because 
Hagar is Abraham's wife, no one can legitimately argue that he is not a totally 
legitimate son.
 
The issue is not legitimacy but a promise that God makes to Abraham:  In Genesis 17:19-21, "Sarah your wife shall bear you a son, and you shall name him Isaac; and I will maintain My covenant with him as an everlasting covenant for his offspring to come.  As for Ishmael, I have heeded you.  I hereby bless him.  
I will make him fertile and exceedingly numerous.  He shall be the father of 
twelve chieftains, and I will make of him a great nation.  But My covenant I 
will maintain with Isaac, whom Sarah shall bear to you at this season next 
year." The covenant was then reaffirmed with Isaac and Jacob, not with 
Ishmael.
 
Second, illegitimate children are those who are born of fornication, adultery, 
or conceived during menstruation (all sinful acts!).  Children of 
Concubines/Slave Women are not illegitimate!  If they were then 4 out of the 12 
sons (tribes) of Jacob would be illegitimate and thusly not be able to inherit 
the land of Palestine.  Because they are legitimate sons of Jacob, they do 
inherit the land of Palestine.
 
 THE TWELVE SONS OF JACOB (Genesis 29:31-30:24; 35:16-26):
· Sons of Leah: Reuben, Simeon, Levi, Judah, Issachar, Zebulon
· Sons of Rachel: Joseph and Benjamin
· Sons of Bilhah (Rachel's handmaid): Dan and Naphtali
· Sons of Zilpah (Leah's handmaid): Gad and Asher 
 
As the brother Ayoub quoted, Qur’an 5:20-21; : “Remember Moses said to his 
people: "O my people! Call in remembrance the favor of Allah unto you, when 
He produced prophets among you, made you kings, and gave you what He had not given to any other among the peoples.  "O my people! Enter the holy land 
which Allah hath assigned unto you, and turn not back ignominiously, for then 
will ye be overthrown, to your own ruin."
 
The other point to be mentioned is that in the Oral "Rabbinic" 
Tradition, it says that when the Torah states that when someone dies and was 
"gathered to his people", it means that they go straight to Paradise.  
Well guess what it says in Genesis 25:17, "This is the account of Ishmael's 
years.  He lived a total of 137 years.  He breathed his last and died, and he 
was gathered to his people."  And that is why according to the Mishna (Oral 
Tradition) Ishmael is in Paradise with Abraham and Isaac.  And that is why one 
of the greatest Rabbi's in the Talmud is named Rabbi Ishmael! because his 
namesake was a righteous person.  No Jew would ever name their kid Esau (the 
evil brother of Jacob, even though he is the grandson of Abraham, because he was evil!)  But because Ishmael was righteous, one of the most famous Rabbis of the Talmud bears his righteous name!  Being a child of Abraham does not mean anything.  The nation that is most cursed in the Torah is the Amalekites, who is the grandson of Esau, and great great grandson of Abraham, and Allah in the Torah (Deuteronomy 25:17-18) commands the Israelites to cause the remembrance of Amalek to exist no longer under heaven, to wipe them out! (and they are Children of Abraham).  From the perspective of the Torah, the covenant of God is solely with the House of Israel (12 Tribes), not with all the Children of Abraham.  And that is why the covenant is reaffirmed with Isaac and not Ishmael and then again with Jacob and not Esau.
 
You have to realize that Judaism is practically identical to Islam, it was only 
the secular atheist socialist racist Zionists living in communistic kibbutz's (communes)and the teachings (of Arab Nationalism) of the Mufti of Palestine in the 1920s and 1930s, Al-Hajj Al-Amin Husseini, who caused all this fitnah.  This racism is not Judaism, it is Jewish Nationalism ( the same as how the PLO and Hamas pervert Islam to suit their desires for nationalism!)  Unfortunately though it was the 2000 years of living under ruthless bloodthirsty Christian Europe including the Inquisition and Pogroms culminating in the Holocaust that created this whole movement.  It did not come out of a vacuum. There are real legitimate reasons why this all happened, it is just sad that 2 people who lived so beautifully in Al-Andalusia and the Ottoman Empire have come down to this!  Let us pray for the peace of Jerusalem as the Psalms of David say!
 
P.S. The other point was that slavery is allowed in both Judaism and Islam, it 
is just regulated as to how to treat a slave.  I hope this clears things up on 
what the Traditional Jewish perspective is.