Ansible synchronize always prepend username#host - ansible

Question

I'm running an ansible 2.3.1.0 on my local machine (macOs) and trying to achieve :
connecting to user1#host1
copying a file from user2#host2:/path/to/file to user1#host1:/tmp/path/to/file
I'm on my local, with host1 as hosts and user1 as remote_user:
- synchronize: mode=pull src=user2#host2:/path/to/file dest=/tmp/path/to/file
Wrong output:
/usr/bin/rsync (...) user1#host1:user2#host2:/path/to/file /tmp/path/to/file
Conclusion
I've been trying different options. I've debugged ansible. I can't understand what's wrong.
Help!
Edit 1
I've also tried adding delegate_to:
- synchronize: mode=pull src=/path/to/file dest=/tmp/path/to/file
delegate_to: host2
It gives:
fatal: [host1]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: Permission denied (publickey,password,keyboard-interactive).\r\n", "unreachable": true}
And also:
- synchronize: mode=pull src=/path/to/file dest=/tmp/path/to/file
delegate_to: user2#host2
Which gives:
fatal: [host1 -> host2]: FAILED! => {"changed": false, "cmd": "/usr/bin/rsync --delay-updates -F --compress --archive --rsh=/usr/bin/ssh -S none -o StrictHostKeyChecking=no --rsync-path=sudo rsync --out-format=<<CHANGED>>%i %n%L host1:/path/to/file /tmp/path/to/file", "failed": true, "msg": "Permission denied (publickey).\r\nrsync: connection unexpectedly closed (0 bytes received so far) [Receiver]\nrsync error: unexplained error (code 255) at io.c(235) [Receiver=3.1.2]\n", "rc": 255}
NB: ssh user1#host1 and then ssh user2#host2 works with ssh keys (no password required)

Answer 1

Please pay attention to this notes from modules' docs:
For the synchronize module, the “local host” is the host the synchronize task originates on, and the “destination host” is the host synchronize is connecting to.
The “local host” can be changed to a different host by using delegate_to. This enables copying between two remote hosts or entirely on one remote machine.
I guess, you may want to try (assuming Ansible can connect to host2):
- synchronize:
src: /path/to/file
dest: /tmp/path/to/file
delegate_to: host2