Ribbentrop, Memoirs. For a similar statement by Steengracht, see IMT, X. Also Goebbels notes after a conversation with Hitler:
'The Fuhrer believes that in this war diplomacy has not so much of a role to play as in former wars' (The Goebbels Diaries, entry for 9th May 1943).
On the other hand Goebbels himself apparently was of another opinion, although doubtless his rivalry with the Foreign Minister played a part in this. So he complained frequently that National Socialist foreign policy had become 'completely solidified and sterile' (entry for 15th November 1943).