Ibn Bukayr (may Allah have mercy on him) narrates from Imam Malik (may Allah have mercy on him) that he heard some scholars say: "Rikaz is the buried treasure of the Jahiliyyah which has not been sought by spending wealth, and for which no great labour has been undertaken. But whatever is sought by spending wealth, or for which great labour is undertaken, so that sometimes it is found and sometimes it is missed, that is not rikaz."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7650