M365 licences - Shared Accounts and terms of service

Hi guys,

I have come across a client that is using shared M365 licenced accounts, e.g. 1 licenced account, used by 5 "real" people. I can strongly assume this is against terms of service, but I cannot find reference of this written anywhere.

Can someone point me to the relevant information so I can reference it so I can resolve this matter with the client and ensure that they are fully compliant.