Musannaf Ibn Abi Shayba 13:2050

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Ash'ath and Dawud, from al-Sha'bi, who said: Umar delivered a sermon, so he praised Allah and extolled Him, then said: "We command you with things that perhaps are not suitable for you, and we forbid you from things that perhaps are not suitable for you. And indeed, the last thing the Prophet, may Allah bless him and grant him peace, entrusted to us was the verse of Riba (usury). The Prophet, may Allah bless him and grant him peace, was taken (passed away) and he had not clarified them to you. It is only Riba and doubt (al-rayb), so leave Riba and doubts."