It says this in two of Vatican II's documents.

The first is in the Dogmatic Constitution on the Church (Lumen Gentium), paragraph 8, where the Church is said to be "sancta simul et semper purificanda" ["at one and the same time holy and always in need of purification".

The second is on the Decree on Ecumenism Unitatis Redintegratio, paragraph 6, where it says the Church is called "ad hanc perennem reformationem" ["to continual reformation"].

This same theme is subsequently taken up in various encyclicals and other papal documents.

There is a particularly interesting example in Benedict XIII's encyclical Deus Caritas Est, in his discussion of the Marx's critique of "charity". BXIII observes that Marx was justified in his criticism, but mistaken in his proposed solution, which was revolution. What BXIII argues is that Marx thought that only one "revolution" would be required; BXIII says it requires a constant revolution ... the same idea: continual need of purification/reform ...