بُلُوغُ الْمَرَامِ
Bulugh al-Maram 624
The Book of Zakah
وَعَنْ أَبِي هُرَيْرَةَ - رضى الله عنه - { أَنَّ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -قَالَ: وَفِي اَلرِّكَازِ: اَلْخُمُسُ . } مُتَّفَقٌ عَلَيْه ِ [1] .
Abu Hurairah (RAA) narrated that The Messenger of Allah (ﷺ) said: “One fifth is compulsory to be paid (as Zakah) on buried Treasure.” Agreed upon.
Sunnah.com reference: Book 4, Hadith 26