What to say to cancel an interview?

OK say a HR calls you from CVS and sets up an interview with for tomorrow...fine, then you get another call from an HR manager from a different company that (reviewed your application and interviewed) from about a week ago and says "we want to hire you, when can you start?" So you tell him your schedule, tell him thank you, looking forward to etc. how do you properly let the HR know, who JUST scheduled you for an interview, kindly?