Bulugh al-Maram #733

'Amro bin Shu'aib narrated on the authority of his father, who reported on the authority of his grandfather (RAA) that the Messenger of Allah (ﷺ) was asked about a treasure that was found by a man in some ruined land, and he said to him, “If you found it in an inhabited village, then you must advertise its having been found. However, if he found it in an uninhabited village, then on it and the rest of the discovered treasure one fifth is payable as Zakah.” Related by Ibn Majah with a reliable chain of narrators.