Midrasch zu Bereschit 21:4

He said to him: “Know of a surety that thy seed shall be a stranger to thee” (ibid.)—from (the time) you will have seed. The Holy One, blessed be He, began to count from the day of his birth.⁹The counting of the 400 years began with the birth of Isaac. And as to the eight days (before) circumcision, since he was the first to be circumcized, it is said: And Abraham circumcised his son Isaac when he was eight days old (ibid. 21:5)—hence the Holy One, blessed be He, gave him his name.

Rabbi said: Abraham did not delay aught with reference to all (things) which He commanded him, as it is said, "And he that is eight days old shall be circumcised" (Gen. 17:12); and when Isaac was born, (and when) he was eight days old (Abraham) brought him to be circumcised, as it is said, "And Abraham circumcised his son Isaac when he was eight days old" (Gen. 21:4). Hence thou mayest learn that everyone who brings his son for circumcision is as though (he were) a high priest bringing his meal offering and his drink offering upon the top of the altar. Hence the sages said: A man is bound to make festivities and a banquet on that day when he has the merit of having his son circumcised, like Abraham our father, who circumcised his son, as it is said, "And Abraham circumcised || his son Isaac" (ibid.).

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).