Musannaf Ibn Abi Shayba 36:481

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, Mujalid ibn Sa'id narrated to us, from al-Sha'bi, who said: The Messenger of Allah (may Allah bless him and grant him peace) wrote to the people of Najran, and they were Christians, that whoever among you engages in usury (riba), then there is no covenant (dhimma) for him.