The Messenger of Allah ﷺ said, "A rider will travel along the side of the valley of Madinah and will say, 'There was once here a large settlement of believers.'"
حدثنا حسن حدثنا ابن لهيعة حدثنا أبو الزبير
أن رسول الله ﷺ قال ليسيرن راكب في جنب وادي المدينة ليقولن لقد كان في هذه مرة حاضرة من المؤمنين كثير
Its isnad is hasan. This wording is unique to Ahmad, and Haythami also graded it hasan, 4/15. It has been mentioned previously with different wordings. See also what follows it, and what follows that.