Edit the file :  /etc/pam.d/sshd

vi /etc/pam.d/sshd

Add this line to the above file :

session optional        pam_exec.so     /path/to/script.sh

Now Create the Script that will send the notification :

vi /path/to/script

Paste The Following content, and adjust the admin email address :

#!/bin/bash
adminEmail="adminemail@yourdomain.com"
[ "$PAM_TYPE" == "open_session" ] || exit 0
        {
        echo "User: $PAM_USER"
        echo "Remote Host: $PAM_RHOST"
        echo "Service: $PAM_SERVICE"
        echo "TTY: $PAM_TTY"
        echo "Date: `date`"
        echo "Server: `hostname -s`"
        } | mail -s "`hostname -s` $PAM_SERVICE login: $PAM_USER" $adminEmail