John Gill Commentary Genesis 21:4

"And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him." — Genesis 21:4 (ASV)

And Abraham circumcised his son Isaac, being eight days old as ,
God had commanded him .
] (Genesis 17:12) , and was the first that we read of that was circumcised on that day, according to the divine precept.

which Abraham strictly observed; for though this was the son of the promise, and of his old age, for whom he had the most affectionate regard, yet he administered this bloody ordinance on him; nor did he defer it beyond the time, and was himself the operator, as it seems; all which shows his strict regard, and ready and cheerful obedience to the command of God.