Musnad Ahmad 32:358

Abu Amir narrated to us, Zuhayr narrated to us, Abd al-Rahman ibn Zayd narrated to me, from his father, that Anas ibn Malik informed him, saying: We went out with the Messenger of Allah, may Allah bless him and grant him peace, to Khaybar. A companion of ours entered a ruin to relieve himself, and he reached for a brick to cleanse himself with, but it collapsed upon him, revealing gold. He took it and brought it to the Prophet, may Allah bless him and grant him peace, and informed him of that. He said: "Weigh it." He weighed it, and it was two hundred dirhams. The Prophet, may Allah bless him and grant him peace, said: "This is *rikaz* (buried treasure), and in it is the fifth."