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.
Rabi'ah bin Abi Abd al-Rahman (may Allah have mercy on him) narrates from more than one of their scholars that the Prophet (peace and blessings be upon him) granted Bilal bin al-Harith al-Muzani (may Allah be pleased with him) the mines of al-Qabaliyyah, which are in the direction of al-Fur'; and nothing but zakah is taken from those mines to this day.
Imam al-Shafi'i (may Allah have mercy on him) said: This is not among what the people of hadith regard as established; and even if they established it, it would contain no narration from the Prophet (peace and blessings be upon him) except his granting of them. As for zakah in mines rather than khums, that is not narrated from the Prophet (peace and blessings be upon him) on this matter. The Shaykh (may Allah have mercy on him) said: It is as al-Shafi'i said regarding the narration of Malik; and it has also been narrated from Abd al-Aziz al-Darawardi from Rabi'ah with a connected (mawsul) chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7636