Sayyiduna Anas bin Malik (may Allah be pleased with him) said: We were with the Messenger of Allah (peace and blessings be upon him), and a companion of ours entered a ruined place to relieve himself, and he saw a nugget of gold. He brought it to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Weigh it." It was weighed and it was two hundred dirhams, so the Messenger of Allah (peace and blessings be upon him) said: "This is rikaz; there is khums in it."
Abd al-Rahman bin Zayd bin Aslam is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7649