What is the Ash'aris' evidence for the obligation of making an exception in matters of faith?

Whoever mandated making an exception in faith did so based on the argument that faith is what a person dies upon, and that faith followed by disbelief is not considered faith, just like one who spoils his prayer before completing it. This is the doctrine of many among the Kullabiyyah and others. The correct doctrine, held by the majority of the Salaf, is that it is permissible to make an exception.