U.S. Hardback edition, Prisoner of Azkaban, chapter 22 ("Owl Post Again"), page 433. "Crookshanks took the order to the Owl Office for me. I used your name but told them to take the gold from my own Gringotts vault."

The House of Black is probably entailed (link from an anonymous poster on cordelia_v's LJ when she asked this). This would mean that Mrs. Black couldn't leave the house to anyone but Sirius.

I'm not sure what that says about Gringott's security. ;) Or who has access to that money now. (It's Sirius' late Uncle Alphard's money--his mother did disinherit him.)