It has been the rule at least since the council of Nicæa (325) that the clergy should stay in the church (or “diocese” as we should call it) of their ordination, cf. Canons of Nicæa xxi. de his qui Ecclesias deserunt et ad alias transeunt, and xxii. de non suscipiendis alterius Ecclesiæ clericis. And we often find Leo insisting on the observance of the rule.