Why does the current Pasuk emphasize Yishmael's pedigree as the son of Avraham?
Gur Aryeh (in 25:19) #1: All of Yishmael's sons came about in Avraham's merit, as Hashem had promised Avraham on the day of his Milah (17:20).
Gur Aryeh (ibid.) #2: As the Pasuk states in Mishlei 17:6 ?The glory of the children is their fathers?. Yishmael's children would also trace their lineage back to Avraham, due to his stature.
Why does the Torah find it necessary to add that Yishmael was the son of Hagar ha'Mitzris Shifchas Sarah?
Rashbam and Ramban (to Bereishis 25:19): To stress that Yishmael was not the real son of Avraham, in contrast to that verse, where it adds "Avraham bore Yitzchak" - since he was his real son. 1
Ha'amek Davar: Regarding Avraham, Yishmael is called a son. Regarding Sarah and Yitzchak, he is called a Ben Shifchah.
Hadar Zekenim (to 25:19): The son of a Nochris is not called your son but her son (Kidushin 68b).
