Musannaf Ibn Abi Shayba 13:199

Al-Fadl ibn Dukayn narrated to us, from Abu Janab and Yazid ibn Mardanbah. One of them said: "He came to us," and the other said: "A letter from Umar ibn Abd al-Aziz came to Abd al-Hamid: 'As for what is before you regarding *al-inah* (a form of usurious sale), it is the sister of *riba* (usury).'"