What do I say to a guy that is rushing our relationship?

I met this guy on an online dating app, and he's amazing and all but he's rushing our relationship and now has brought up marriage. I am completely freaking out because I barely know this guy and we have only been talking for about a week or so. I told him more than once that we need to slow down but he seems to not get it. What do I say so he understands me completely?

What do I say to a guy that is rushing our relationship?
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