In Cloud9 I do:
$ git push -u origin --all
The authenticity of host 'bitbucket.org (131.103.20.168)' can't be established.
RSA key fingerprint is 97:8c:1b:f2:6f:14:6b:5c:3b:ec:aa:46:46:74:7c:40.
Are you sure you want to continue connecting (yes/no)?
I added the ssh-key from cloud9 to Bitbucket. Shouldn't that be enough to have Bitbucket authenticated by Cloud9?
No. When you'll first connecting to bitbucket, ssh client on your machine will store RSA fingerprint in file called known_hosts. Then before each connection server fingerprint will be validated with stored one (to avoid man-in-the-middle attack).
So - you need to accept this fingerprint only once (if you're diligent you should compare it with fingerprint provided by bitbucket).
If your key is added, you might be missing this important step...
When we get the prompt Are you sure you want to continue connecting(yes/no)? then we should type yes before hitting the return/Enter key.
Good Luck.
来源：https://stackoverflow.com/questions/26664551/the-authenticity-of-host-bitbucket-org-131-103-20-168-cant-be-established