Musannaf Ibn Abi Shayba 36:200

Jarir narrated to us, from Mughira, from Ibrahim, who said: The Messenger of Allah (peace and blessings of Allah be upon him) set the ransom for an Arab on the day of Badr at forty awqiyah, and he set the ransom for a mawla (freedman/client) at twenty awqiyah. The awqiyah is forty dirhams.