Genesis 21:4
"And Abraham circumcised his son Isaac being eight days old, as God had commanded him."
Cross References (ranked by relevance)
And he gave him the covenant of circumcision: and so Abraham begat Isaac, and circumcised him the eighth day; and Isaac begat Jacob; and Jacob begat the twelve patriarchs.
What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.
And when eight days were accomplished for the circumcising of the child, his name was called JESUS, which was so named of the angel before he was conceived in the womb.
And they were both righteous before God, walking in all the commandments and ordinances of the Lord blameless.
And when a stranger shall sojourn with thee, and will keep the passover to the LORD, let all his males be circumcised, and then let him come near and keep it; and he shall be as one that is born in the land: for no uncircumcised person shall eat thereof.
And it came to pass, that on the eighth day they came to circumcise the child; and they called him Zacharias, after the name of his father.
And in the eighth day the flesh of his foreskin shall be circumcised.