ليس بعرض
God is not an accident.
In the classical terminology, an accident (ʿaraḍ) depends on a subject or locus in order to subsist; color, motion and rest are discussed under this heading. Calling God an accident would make divine existence dependent on an underlying bearer, whereas the Necessary Being cannot depend on another for existence.
The argument is not merely a dispute about vocabulary. It concerns the incompatibility of self-subsistence with dependence on a locus. What explains the being of contingent things cannot itself be made dependent on a creaturely substrate.