In the celebrated book of Hilbert and Cohn-Vossen, the following sentence appears (p. 230):
Bending is impossible in the case of all closed convex surfaces, such as, for example, the ellipsoids. It is likewise impossible to bend any convex surface having boundary curves, provided each boundary curve has the property that at all of its points the tangent plane is the same. An example of such a surface (with two boundary curves) is the convex part of the torus.
I couldn't find any hint of a proof of the second part of the sentence. I know how to prove that such surfaces are infinitesimally-rigid, but as far as I can tell, Hilbert and Cohn-Vossen refer to global rigidity.