Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that a man came to the Prophet (peace and blessings be upon him) with five uqiyahs and said: "O Messenger of Allah! I obtained this from a mine; take the zakah from it." He (peace and blessings be upon him) said: "There is nothing (due) in it," and he returned it to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7644