Humran narrates: Sayyiduna Uthman Ghani (may Allah be pleased with him) called for water for ablution; he was about to go out for the prayer. This was on a cold night. I brought him water, and he washed his face and both arms. I said: This much is enough for you; it is very cold tonight. He said: I heard the Prophet (peace and blessings be upon him) say: "Whenever a servant performs ablution thoroughly, Allah Almighty forgives his past and future sins."
Al-Bazzar recorded it, and its narrators have been declared trustworthy; this hadith will be hasan (good), if Allah wills.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1216