It’s mentioned in Dione Lucas’ (Hitler’s personal chef) book “The Gourmet Cooking School Cookbook” https://www.amazon.com/Dione-Gourmet-Cooking-School-Cookbook/dp/0517370611
She worked as a chef at a hotel in Hamburg where Hitler dined fairly often. Of this customer, and her recipe for stuffed squab, she wrote: “I learned this recipe when I worked as a chef before World War II, in one of the large hotels in Hamburg, Germany. I do not mean to spoil your appetite for stuffed squab, but you might be interested to know that it was a great favorite of Mr. Hitler , who dined at the hotel often. Let us not hold that against a fine recipe, though.”