Adverse Action Letter
The Fair Credit Reporting Act, as amended by Dodd Frank in 2010, requires creditors (that's you, landlord) to notify consumers (the tenants) when adverse action is taken. Adverse action in the case of landlord-tenant relations means "rejecting a tenant because of something on their credit report or because of their credit score." The form of notice is regulated by law and called an "adverse action letter" or "adverse action notice."
This notice must provide contact information for the reporting agency issuing the report, and if the score itself was used in your decision (even if all you did was look at it), you must also disclose the tenant's credit score and teach them about credit scores. In either case, you must also inform them of their rights to dispute the report. But they don't have the right to dispute your having rejected them. Your rejection can be final.
You must be very careful to follow the Fair Credit Reporting Act law and not just this form. If you obtain information from a consumer reporting agency not listed here, you must provide all the same information as what you see for Experian, etc. If any of you still use bad check reports or a source of eviction information besides MassCourts.org, you must provide their information, as well (if they're the reason you're rejecting the tenant.)