what's the safe way to use rsync without password prompt in shell script

I want to sync a remote directory to local with rsync command, my local server is redhat enterprise linux 6.5.

I googled, but only found tutorials how to set up ssh key with empty passphrase.

I followed one tutorial from here , and it worked, calling rsync will not ask for password any more.

rsync -avzhe --min-size=1 ssh user@host:/somedir ~/backup

but, our system is in production environment, I think it may be not good to use ssh-key with empty passphrase from the security perspective. Correct me if I am wrong, but if I am right, how to safely use rsync without password prompt ?

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