Génesis 16:11 Midrash: Legends, Pirkei DeRabbi Eliezer & Bereshit Rabbah

Abraham, modest and unassuming as he was, was ready to do justice to Sarah, and he conferred full power upon her to dispose of Hagar according to her pleasure. He added but one caution, "Having once made her a mistress, we cannot again reduce her to the state of a bondwoman." Unmindful of this warning, Sarah exacted the services of a slave from Hagar. Not alone this, she tormented her, and finally she cast an evil eye upon her, so that the unborn child dropped from her, and she ran away. On her flight she was met by several angels, and they bade her return, at the same time making known to her that she would bear a son who should be called Ishmael--one of the six men who have been given a name by God before their birth, the others being Isaac, Moses, Solomon, Josiah, and the Messiah.

Balaam said: Of the seventy nations that the Holy One, blessed be He, created in His world, He did not put His name on any one of them except on Israel; and since the Holy One, blessed be He, made the name of Ishmael similar to the name of Israel, woe to him who shall live in his days, as it is said, "Alas, who shall live when God establisheth him?" (Num. 24:23).

How do we know (this with reference to) Isaac? Because it is said, "And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac" (Gen. 17:19). Why was his name called Isaac? Because Yad (the first Hebrew letter of Isaac indicates) the ten trials wherewith our father Abraham was tried; and he withstood them all. Zaddi (the second letter indicates) the ninety (years), for his mother was ninety years (at the birth of Isaac), as it is said, "And shall Sarah, that is ninety years old, bear?" (Gen. 17:17). Cheth (the third letter points to) the eighth (day), for he was circumcised on the eighth day, as it is said, "And Abraham circumcised his son Isaac, being eight days old" (Gen. 21:4). Ḳuf (the fourth letter of the name marks) the hundred (years), for his father was an hundred years old (at Isaac's birth), as it is said, "And Abraham was an hundred years old" (Gen. 21:5).