Cited by Gerhard Ebeling, Luther. Introduction à une réflexion théologique, Genève, Labor et Fides, p. 96-97.

God does not give a person his Spirit or his grace except through or with his existing external Word. This is our safeguard against the enthusiasts, in other words, the souls who flatter themselves that they receive the Spirit independently of the Word and prior to it, and who, as a result, evaluate, interpret and extend Scripture and the spoken word as they wish.....It is why we have the duty and obligation to maintain that God wishes to enter into relationship with us, men, only through his external Word and through the sacraments. Anything that is said of the Spirit independently of that Word and the sacraments is the work of the devil.