unable to delegate_to in playbook

[Vivek Aggarwal]

i have a playbook in which there one particular task which i want to run only on one host & i m tried that using "delegate_to"  but while executing it is showing ssh error.

SSH user is showing as NONE. PLease suggest what wrong i m doing

<30.0.7.137> ESTABLISH SSH CONNECTION FOR USER: None

+++++++++++++++++++++++++

 #- include: "get_aws_ips.yml"

 - hosts: all

   remote_user: ec2-user

   gather_facts: no

   tasks:

   - name: find master key md5dum

     command: "hostname"

     delegate_to: "30.0.7.137"

     ignore_errors: yes

++++++++++++++++++++++++++++++++++++++++++