From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sacred Ash is a healing item found in all games since Generation II. Unlike other items in general, which work on only one Pokémon, the Sacred Ash, when used, revives all Pokémon in a Trainer's party to full HP and PP. It is very rare, and can only be used once, making it a very popular item for cloning glitches.
The Sacred Ash is typically found only once per game, and will often have some relation to Ho-Oh.
In Pokémon Gold, Silver, and Crystal, the wild Ho-Oh that appears at the Tin Tower will be holding a Sacred Ash, as will any other Ho-Oh encountered in the wild through hacking.
In Pokémon FireRed, LeafGreen, and Emerald, the Sacred Ash is found on Navel Rock, if the player uses the Itemfinder after capturing or defeating Ho-Oh.
Another Sacred Ash can be found in the Battle Pyramid; however, it cannot be kept.
By defeating Battle CD 46, the player can earn another Sacred Ash as a prize in Pokémon XD.
Currently, the only way to get a Sacred Ash in Pokémon Diamond, Pearl, and Platinum is by having a Pokémon hold it in Generation III and sending it through Pal Park. Wild Ho-Oh, which can only be encountered with a cheating device, will always be holding this item.
In Pokémon HeartGold and SoulSilver, the item is held by The ho-oh found on top of tin tower, like in Generation II