As an AWS Certified Developer Associate currently working on designing and deploying cloud infrastructure, you are tasked with writing a CloudFormation template using YAML. The specific requirement is to create an EC2 instance and an RDS (Relational Database Service) resource within this template. Upon successful creation of these resources, you also need to configure the template to output the connection endpoint of the RDS database.

Which intrinsic function should you use to return the necessary connection endpoint value?