Need advice: Made an insensitive comment to someone with a stutter. How to apologize?

Need advice: Made an insensitive comment to someone with a stutter. How to apologize? I was making friends with someone new and had spoken for a bit but didn’t realize they had a stutter and made an insensitive comment about them forgetting their number when asking about it. I want to apologize but not sure about the best way to approach it. Thank you in advance.