Jarir bin Rabah narrates from his father that they found a grave in al-Mada'in, and in it they found a man wearing clothes woven with gold, and they also found wealth with him. They came to Ammar bin Yasir (may Allah be pleased with him), and he wrote to Umar (may Allah be pleased with him) about it. Umar (may Allah be pleased with him) wrote back to him that he should give it to them and not take it away from them.
The Shaykh said: If they found it in dead (unowned, uncultivated) land, then they own it and there is khums in it; it seems that it did not reach the nisab, or he left that to them so that they would pay it themselves. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7654