Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) said: When some men among us came out after relieving themselves, they would wash away the traces (with water), so this verse was revealed concerning them: "In it (the mosque) are men who love to purify themselves." [Surah al-Tawbah: 108]
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3778