Where does EJ say he found Stahlgewitter in Edda?

[Rebing]

Your are right, Bertil, if EJ says he got Stahlgewitter from the Edda it must be
so, he used to be very accurate in such philological-bibliographical matters. So
halali again, lets us go on looking for the source! But where exactly in his
writings did the Master say he took it from the Edda? Hälsningar, Günter