A credential of the type "SSH Username with private key" with the RSA private key of the EC2 key pair. You can use the AWS Console or any other tool of your choice to generate the key pair. The username is used, and the credential should not have a passphrase.

An RSA private key for EC2 is a text that starts with "-----BEGIN RSA PRIVATE KEY-----". You normally generate it via using either the AWS Management Console, the "ec2-add-keypair" tool or a GUI tool like ElastiFox.