There is some debate to it, but the official story is that the people stationed in the bunker with Hitler tried to burn the bodies of Hitler and Eva Braun after finding them dead. Russians were bombing the area. Afterwards, Russians came in, discovered the bodies, and buried them in Rathenau. A few months later, the bodies were moved to Madgeburg. In 1970, as the garrison they were at was being abandoned, KGB then further burned the bodies and spread the ashes.

Part of why there is debate, is because over the years the Soviets claimed different stories. For a few years, Stalin maintained that Hitler was alive and being protected by the western Allies. After that, the Soviets story would reflect what ever they felt best suited them at the time.

There was a fragment saved that was purported by the KGB to be from Hitler, but some DNA testing showed it wasn't his. Russians still claim that it is his.