The Prophet ﷺ said that this verse was revealed concerning the people of Quba, "In it are men who love to purify themselves" [Tawbah 9:108]. He explained that they used to clean themselves with water after relieving themselves, so this verse was revealed about them.
حدثنا محمد بن العلاء أخبرنا معاوية بن هشام عن يونس بن الحارث عن إبراهيم بن أبي ميمونة عن أبي صالح
عن النبي ﷺ قال نزلت هذه الآية في أهل قباء {فيه رجال يحبون أن يتطهروا} قال كانوا يستنجون بالماء فنزلت فيهم هذه الآية
[Hasan] Tirmidhi narrated it in tafsir of the Quran, chapter on Surah Tawbah, no. 3100, from Ibn Ala through this route, and Ibn Majah narrated it, no. 357. Tirmidhi said, "Gharib." The hadith has corroborating narrations in Ibn Majah, no. 355, and elsewhere.