/etc/ssh/sshd_config: 

#Subsystem sftp /usr/lib/openssh/sftp-server

Subsystem sftp internal-sftp

Match group chrooted

    ChrootDirectory /www/sites

    X11Forwarding no

    AllowTcpForwarding no

    ForceCommand internal-sftp

/etc/passwd:

testuser:x:1002:1002::/www/sites/testuser:/usr/lib/sftp-server