"In contrast to a widespread opinion among systematic theologians, the formula ecclesia semper reformanda is not attested in Protestant theology of the sixteenth century. It is most likely that the idea that the Reformation should not remain simply a one-time event, but that it is an ongoing duty of the church, goes back to those Dutch pastors who should be placed in the context of the prehistory of Pietism."
Oxford encyclopedia of the Reformation (1996), s.v. "Reform," by Konrad Repgen, as trans. Robert E. Shillenn (vol. 3, p. 395), underscoring mine.
I have not verified this personally.