Musannaf Ibn Abi Shayba 13:1409

Abu Bakr narrated to us, he said: Ibn Abi Za’idah and Waki’ narrated to us, from Abdullah ibn Humayd, from his father, from his grandfather, that Umar ibn al-Khattab gave him the wealth of an orphan for *mudarabah* (profit-sharing partnership). He sought profit in it and succeeded, so he divided the surplus with him, then they parted ways.