Musnad Ahmad 350
It was narrated from Ibn al Musayyab that `Umar (رضي الله عنه) said:
One of the last verses to be revealed was the verse on riba, and when the Messenger of Allah (ﷺ) died, he had not explained it. So avoid riba and any dubious matter.
حَدَّثَنَا إِسْمَاعِيلُ، عَنِ ابْنِ أَبِي عَرُوبَةَ، عَنْ قَتَادَةَ، عَنِ ابْنِ الْمُسَيَّبِ، أَنَّ عُمَرَ، رَضِيَ اللَّهُ عَنْهُ قَالَ إِنَّ مِنْ آخِرِ مَا أُنْزِلَ آيَةُ الرِّبَا وَإِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ تُوُفِّيَ وَلَمْ يُفَسِّرْهَا فَدَعُوا الرِّبَا وَالرِّيبَةَ‏.‏