Sunan Ibn Majah, Hadith 2510 ⓘ
It was narrated from Ibn`Abbas that the Messenger of Allah said: “One fifth is due on buried treasure.”
حَدَّثَنَا نَصْرُ بْنُ عَلِيىٍّ الْجَهْضَمِيُّ، حَدَّثَنَا أَبُو أَحْمَدَ، عَنْ إِسْرَائِيلَ، عَنْ سِمَاكٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " فِي الرِّكَازِ الْخُمُسُ " .
also referenced under: Book 18, Hadith 2606 (In-Book) & Volume 3, Book 18, Hadith 2510 (DarusSalam)