When we create a sandbox we have several WorkFlowOutbound message-setting from which the url needs to be changed.
As a manual process this is hard work, so I tried with Salesforce DX to retrieve this with the following command :
sfdx force:source:retrieve --metadata WorkflowOutboundMessage

But I don't receive any result. 
I also already tried with Illuminated Cloud which shows the info when indicating which meta-data to retrieve, but in the end it is not retrieved at all