Gen 22:18, Gal 3:16

Abraham was waiting for the coming of Christ. God promised to send Christ as a descendant of Abraham. (Gen 22:18, Gal 3:16)

Gen 22:18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.

Gal 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.

So, when Abraham saw the birth of his son Isaac, he rejoiced in anticipation of the birth of Christ. (Jn 8:56)

Jn 8:56 Your father Abraham rejoiced to see my day: and he saw it, and was glad.