Vinogradov, V. K. (2005). Hitler's Death: Russia's Last Great Secret from the Files of the KGB. Chaucer Press. ISBN 978-1-904449-13-3.

If you trust the files of the KGB, the ashes and whatever else remained, were tossed into the Biederitz River. Aside from those records, there are no other records I know of that mention what happened to his remains.

If someone has another citation, please give it!