Musannaf Ibn Abi Shayba 28:388

Abdullah ibn Numayr narrated to us, from Sufyan, from al-Aswad ibn Qays, from Nabih, who said: Ibn Umar bought two camels and placed them among the camels of the Sadaqah (charity). They became fat, grew large, and their condition improved. Umar saw them and disapproved of their condition. He said, "To whom do these belong?" They said, "To Abdullah ibn Umar." He said, "Sell them, take your capital, and return the surplus to the public treasury (Bayt al-Mal)."