disable rubocop error 'Use find_by instead of where.first' for rails 3.2 project

I am using rubocop that shows error Use find_by instead of where.first. My rails version is 3.2 that does not support find_by method that's why I am trying to disable this. I have tried using following configuration with in my .rubocop.yml file but did not work

Rails: Enabled: false

Does anyone has any idea how can I disable above error for rubocop? Any kind of help is appreciated. Thanks.