It looks like there is an automated ssh scanner on that host (or that server could be compromised) and it's trying to bruteforce or exploit randomly. You can check your /var/log/secure & messages log files to tell if that host was able to gain access to your server or not. It's recommended to tweak your sshd configs and consider changing your ssh port to something else other than the default 22.
I strongly suggest you install ConfigServer Security Firewall along with Mod_Security. I am not trying to pimp it, I happened to be getting port scanned off the map until I figured out how to use Configserver.
rm -fv csf.tgz
tar -xzf csf.tgz
Later go to whm Plugins
Now you need to configure it, takes about an hour to get through everything correctly the first time around, it will guide you on how to create a very secure environment and it is 100% free to use.