You will need to add your Lambda function to your VPC by clicking on the function in the Lambda console, clicking on Configuration, and then clicking on VPC.
Next, go to the EC2 console, and click on Security Groups in the Network & Security menu in the left-hand side-bar. Create a group for your lambda function and name it something descriptive. Then configure the outbound rule for the Lambda function security group to have the RDS security group as its destination.
Next, configure the inbound rule for the RDS security group to have the Lambda function security group as its destination.