Midrash su Salmi 71:14

And thou shalt command the children of Israel (Exod. 27:20). May it please our masters to teach us: At what age must an infant be circumcised? So do our masters teach us: An infant must be circumcised on the eighth day. Why? Because our patriarch Isaac was circumcised on that day. R. Simeon the son of Yohai said: Observe that though nothing is more precious to a man than his own son, he is required to circumcise him. Why must he do that? R. Nahman the son of Samuel said: He does so in order to fulfill the will of his Creator, and even though he witnesses his son’s blood flowing from the circumcision, he welcomes it joyfully. R. Hanina declared: And what is more, he spends his money to celebrate the occasion, something which he was not commanded (to do). Why is that so? Because Scripture states: But as for me, I will hope continually, and will praise Thee yet more and more (Ps. 71:14). Not only that, but a man even borrows money and goes into debt (if need be) to celebrate on that day.

And Abraham took another wife (Gen. 25:1). Scripture states elsewhere in allusion to this verse: But as for me, I will hope continually and will praise Thee, yet more and more (Ps. 71:14). R. Yudan stated: It is written: Only he shall not multiply horses to himself (Deut. 17:16). When Solomon was crowned king, he did not possess a single horse, and was compelled to ride upon an ass, but later on he obtained numerous horses.

R. Hunya said: It is written: And in the eighth day the flesh of his foreskin shall be circumcised (Lev. 12:3). This indicates that a man should even go into debt in order to make the circumcision a day of rejoicing. Hence it says: And will praise Thee, yet more and more (Ps. 71:14). The rabbis maintained that this verse speaks of Abraham. He said to the Holy One, blessed be He: “You told me: For in Isaac shall seed be called unto thee (Gen. 21:12). You have multiplied Your blessings upon me, and I have borne many sons.” Hence it is said: And Abraham took another wife.