Musannaf Ibn Abi Shayba 28:169

Hushaym narrated to us, from Husayn, from someone who witnessed [the Battle of] al-Qadisiyyah, who said: While a man was bathing, the water uncovered for him from the dirt a brick of gold. He came to Sa’d ibn Abi Waqqas and informed him. He said: "Place it among the spoils of the Muslims."