Because of this saying of the Prophet (peace and blessings be upon him): "The mine (the blood of one who falls into it and dies) is without liability, and in rikaz (buried treasure) there is one-fifth (khums)."
So he (peace and blessings be upon him) distinguished between the two in what he mentioned, and he attributed the khums (one-fifth) to rikaz.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / Q7636