What statute or case law says that there cannot be a lawsuit without a written/oral agreement?

I'm a first year law student, and I have to do legal analysis on a contract case. I know that to take someone to court, there has to be a written/oral agreement. There has to be a contract. But in which statute/case law is this mentioned? My entire analysis is based on this. X wants to sue Y, but there is no contract between them. My argument is that X cannot sue Y because there is no written/oral agreement, but where can I find that law?