Is lying about having performed the prayer—such as when a worshipper claims to have performed it but has not—considered major disbelief (kufr akbar) or merely a grave sin?

Lying is forbidden (haram), but its perpetrator does not become an unbeliever (kafir) because of it. And whoever says, "I prayed," while he is lying, does not become an unbeliever due to his lie, nor does he become an unbeliever in a major sense (kufran akbar) for abandoning prayer, even though abandoning it is one of the greatest major sins.