Source: http://www.howtogeek.com/121650/how-to-secure-ssh-with-google-authenticators-two-factor-authentication/

Also, for CentOS: http://www.tecmint.com/ssh-two-factor-authentication/

Want to secure your SSH server with easy-to-use two-factor authentication? Google provides the necessary software to integrate Google Authenticator’s time-based one-time password (TOTP) system with your SSH server. You’ll have to enter the code from your phone when you connect.