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).