Mention the user name of the user who wrote the comment.
Comments are not meant for eternity and everybody who writes a comment here must know that it is subject to deletion once it has served its cause. If you integrate a comment into a post, the comment should be flagged as no longer needed. Therefore, there is little point in attempting to link the comment directly since it will disappear.
Thus, the name and the fact that it was a comment is all the durable information you have. Present this.
Please don’t, however, mention the user with an @ preceeding their name. In comments, this @ serves a pinging purpose, but it is not possible to ping users from the body of a post. There is also no need to highlight that this is a username since anybody with a decent understanding of English should be able to realise that we are dealing with a name in this context. An added @ disrupts the flow of text, however. It is therefore typographically desirable to leave it out.