Resources:
  ...
  MyBucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      BucketName: 'my-bucket'
  PermissionForS3ToInvokeMyLambda:
    Type: 'AWS::Lambda::Permission'
    Properties:
      Action: 'lambda:InvokeFunction'
      FunctionName: !GetAtt MyLambda.Arn
      Principal: 's3.amazonaws.com'
      SourceArn: !Ref MyBucket
  ...