Angular: get attribute from directive

I have no idea why, but:

@myApp.directive 'myDirective', () ->
return {
    restrict: 'E',
    scope: {
        source: '='
    },
    link: (scope, element, attrs) ->
        console.log scope.source
}

<my-directive source="foobar"></my-directive>

returns undefined. The thing which confuses me is that in my other directive, print-user, everything works fine.

return {
restrict: 'E',
scope: {
  user: '=',
  showName: '=',
  showAvatar: '=',
  avatarSize: '='
},
templateUrl: 'templates/partials/print-user.html',
link: (scope, element, attrs) ->
  scope.tooltip = scope.user.username
}

Here I'm able to get the user object within the template by {{user}}.

Within myDirective I could get the source attribute by attrs.source - but why is it working within my user directive?

EDIT / Solution Thanks to Aleksandar Bencun: using <my-directive source="'foobar'"></my-directive> (additional single quotes) solved the problem.