4 And Abraham circumcised his son Isaac being eight days old, as God had commanded him.
5 And Abraham was an hundred years old, when his son Isaac was born unto him.
6 And Sarah said, God hath made me to laugh, so that all that hear will laugh with me.
7 And she said, Who would have said unto Abraham, that Sarah should have given children suck? for I have born him a son in his old age.
The record once again makes abundantly clear that Abraham immediately followed all of the instructions God had given him, and circumcised his son when he was eight days old, the exact age that the Lord had specified in His commandments. By this he welcomed his son into the covenant that God had established, making Isaac ready to receive all the promised blessings.
As for Sarah, she recalled the moment when God said she would have a son in her old age and she laughed in incredulity. The Hebrew word used here for “laugh” is the exact same as the one used in that earlier passage, which means to make sport or play. Sarah was observing that all the world would be in on the joke now that the promise had been fulfilled. A joke that she was happy to have at her own expense!