I just accepted a job offer and got my start date! How to politely reject an interview from a company that a senior leader referred me to?

I just got a job offer at company X and got my start date. All background checks are completed. When I was job hunting, I asked a college alumni who is a senior leader at a company Y to refer me. It took him 3 weeks to get back to me and by that point I have already accepted another position.

Question:

How do I politely tell him that I accepted a position at another company, but not burn any bridges so in case in the future I want to work still work there, I can ask him for another referral?

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