Musannaf Ibn Abi Shayba 27:831

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 *uqiyahs*, and he set the ransom for a *mawla* (freed slave/client) at twenty *uqiyahs*. And the *uqiyah* is forty dirhams.