Another note to myself.
In case of
> ssh -T user@host

Outputs this type of error.
(Most likely because you forgot to "ssh-add /path_to_your_key")
> Permission denied (publickey).

Delete "config" and "known_host" file from ~/.ssh directory (your default key location)

And if "ssh-add" outputs this error.
> Could not open a connection to your authentication agent.

Execute this command
> eval "$(ssh-agent)"

Then again. Re-add your key by
> ssh-add /path_to_your_key