When praying on behalf of Yishma'el, why did Avraham add the word "Lefanecha""?
Rashi: The word "Lefanecha," when it refers to Hashem, means "with Yir'as Shamayim." 1
Targum Yonasan: It means that "he should (live and) worship You."
Ramban: Avraham was Davening that Yishmael should live, and that his offspring should live as long as the world lasts.
Seeing as Hashem just promised Avraham that he would have a child from Sarah, why did he Daven for Yishma'el?
Ramban: Until now Avraham believed that Hashem's promise that his own child (and not Eliezer) would inherit him, referred to Yishmael. Now that Hashem informed him that in fact, it referred to Yitzchak, he was afraid that Yishmael would die. Therefore he declared that he only accepted the new Berachah "if Yishmael's seed will live permanently."
Rashi: Avraham felt, I am not worthy to receive such reward 1 (a son from Sarah).
Ma'ase Hashem: I.e. Avraham did not want to be like one who serves in order to receive reward.
QUESTIONS ON RASHI
Rashi writes: "If only Yishma'el will live; [for] I am an unworthy of such a reward!" How is this implied in the Pasuk?
Gur Aryeh: The literal meaning would be that Avraham feared that Yishma'el would perish; but why would that be? Rather, Avraham was expressing his feeling of unworthiness of this gift. 1
Rashi writes: "[If only Yishma'el] would live before You - i.e., as G-d fearing." Ramban asks
Gur Aryeh: Avraham could not have asked directly that Yishma'el should fear Hashem. Fear of Hashem is the task of every individual. Avraham merely asked that Yishma'el live, and thus have the opportunity to serve and fear Hashem; and this is what Hashem granted.