Placing stones on a matzeivoh

Some have a custom to place stones/pebbles on a matzeivoh they visit at the cemetery.

From where does this come from, and is it proper/acceptable/recommended according to minhag Ashkenaz hamuvhak?

Litvak

Michael wrote: The Minhag is to place stones/pebbles, because this is Kovod Hames, to show he was not forgotten and people visited his grave.

This is probably also to show the dearly departed that he was not forgotten, as according to Masoras Ashkenaz, the dead leave their graves from time to time. See Yerushaseinu #3, p. 177-236.

MPerlman