(server)$ cat << EOF >> .ssh/authorized_keys
command="rsync",no-pty,no-port-forwarding (paste your public key here)
EOF

(server)$ tail -1 .ssh/authorized_keys
command="rsync",no-pty,no-port-forwarding ssh-rsa AAAAB3..blah..blah..HhcvQ== you@example.com

rsync -av -e 'ssh -i ~/.ssh/id_rsa_rsync' local/files* server:

# this entry is used for normal logins to "server":
Host server
  IdentityFile ~/.ssh/id_rsa

# this entry is used for rsyncing to "server" without a passphrase:
Host server-rsync
  Hostname server
  IdentityFile ~/.ssh/id_rsa_rsync

rsync -av local/files* server-rsync: