All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It seems that the questioner is referring to the narration reported by Al-Bukhari that the Prophet said about Makkah: "Its thorns are not to be cut, its game is not to be molested, anything which falls in it is lawful only to the one who announces it" (i.e. it is not lawful for one who finds something that fell on its streets to pick it up except to announce it to people so that its owner recognises it and retrieves it). Then Ibn 'Abbaas said: "Except the tree called Al-Ithkhir, O Messenger of Allaah, we use it in our graves and houses. He said: "Except Al-Ithkhir."
The leaves of this kind of tree is used in making roofs, placing them over the wood, and they are used as well to cover the corpses in graves, and blacksmiths and goldsmiths use them too. Imaam Al-Bukhari wrote a chapter about this narration, entitling it: "The Book of Al-Ithkhir and herbs in graves".
Allaah knows best.