How should I cancel my interview?

I started applying for a few jobs last week just to see what was out there. I actually got a response and had a phone interview yesterday for one. I was invited to do an in person interview next week.

The thing is, I don't really think I want the job anymore. I think it'd be unprofessional to cancel the day after I had my phone interview, but how should I tell them? Should I just tell them I found another opportunity and thank them for considering me? How can I word it?

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