Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) says: When our people went to relieve themselves, they would afterwards wash away the traces of excrement, and this verse was revealed concerning them: "In it (the mosque) are men who love to purify themselves thoroughly."
Imam al-Tabarani recorded this narration; its chain contains a narrator, Abu Bakr bin Abi Sabrah, and he is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1060