When running rsync you get an error a bit like this:

  The authenticity of host '192.168.1.10 (192.168.1.10)' can't be established.
  ECDSA key fingerprint is 65:76:dc:ba:74:d6:c7:75:7f:21:58:c6:66:42:23:7d.

Just SSH to the machine from the command line and your key will be added to the target host. You only need to do this once.