Musnad Ahmad 2:268

Isma'il narrated to us, from Ibn Abi 'Arubah, from Qatadah, from Ibn al-Musayyib: That 'Umar said: "Indeed, among the last of what was revealed was the verse of Riba (usury/interest). And the Messenger of Allah (peace and blessings of Allah be upon him) passed away and did not explain it. So, leave Riba and that which causes doubt (al-ribah)."