Musnad Ahmad 2:165

Yahya narrated to us, from Ibn Abi Arubah, Qatadah narrated to us, from Sa’id ibn al-Musayyib, who said: Umar said: "Indeed, the last of what was revealed of the Qur’an 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)."