Midras Kejadian 17:19: Midrash Tanchuma, Mekhilta DeRabbi Yishmael, Pirkei DeRabbi Eliezer & Bereshit Rabbah

Now these are the names (Exod. 1:1). The names of the Israelites were most appropriate for them. You find, in fact, that the Holy One, blessed be He, assigned their names to them. He gave Abraham a son, and He said to him: And thou shalt call his name Isaac (Gen. 17:19). The yod in his name equals ten, the tzaddi, ninety, the het, eight, and the kuf, one hundred, totaling two hundred and eight in all. But the Israelites were forced to remain in Egypt only two hundred and ten years⁸Two years were added because he had asked the baker to intervene in his behalf, thus indicating a lack of faith that God would deliver him. It took two years for the baker to do so. even though the Holy One, blessed be He, had told Abraham: And shall serve them; and they shall afflict them four hundred years (ibid. 15:13).

Similarly, (Genesis 21:1) "And the L–rd remembered Sarah (for motherhood) as He had said." "And where did He say it? (Ibid. 17:19) "And G–d said: But Sarah your wife will bear, etc."

The name of Yitzchak was not changed, for he was thus (originally) called by the Holy One Blessed be He. There are three who were named by the Holy One Blessed be He — Yitzchak, Shlomoh, and Yoshiyahu. Yitzchak — (Ibid. 17:19) "But Sarah your wife will bear a son for you and you shall call his name Yitzchak." Shlomoh (I Chronicles 22:9) "for Shlomoh will be his name." Yoshiyahu (I Kings 13:2) "A son will be born to the house of David. Yoshiyahu will be his name." Others say: Also Yishmael among the gentiles.