When a bug posted by QA is rejected by Dev and Dev says is not a bug. How to convince Dev that it is a valid bug?

case1: Requirement exists for the reported bug

If Dev says it is an invalid issue, then the tester need to
give him reference no of the requirement.

case2: Requirement not exists for the reported bug.

Sometimes we may encounter some issues while testing which
do not have requirements. In that case we still have right
to submit a bug with issue category as requirements. If the
developer says that it is an invalid issue then we have to
move that bug to business analyst asking his comments. Once
we receive the comments from business analyst we will move
back the bug to developers court.