There's no direct mentions in the manga, only implied ones. The clear, direct statement on this is in Perfect Guidebook 2, in the page with the Piramid of Hierarchy of Amestrian Military and who's where in that "Piramid". The quote stated there is: "General Grumman: Grandfather of Lieutenant Riza Hawkeye from her mother's side, Grumman is the head of the Eastern Headquarters... etc" So, this clarifies why Riza doesn't bear the last name "Grumman", since the General was the father to her mother.