'Amr b Sa'id b al-As reported:
I was, with Uthman that he called for ablution water and said: I heard Allah's Messenger (ﷺ) say: When the time for a prescribed prayer comees, if any Muslim perform ablution well and offers his prayer) with humility and bowing, it will be an expiation for his past sins, so long as he has not committed a major sin; and this applies to for all times.
Reference : Sahih Muslim 228
In-book reference : Book 2, Hadith 10
USC-MSA web (English) reference : Book 2, Hadith 441
(deprecated numbering scheme)
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Humran, the freed slave of 'Uthman reported:
I brought for Uthman b. 'Affan the ablution water. He performed ablution and then said: Verily the people narrate from the Messenger of Allah (ﷺ) a hadith. I do not know what these are. but (I know this fact) that I saw the Messenger of Allah (ﷺ) perform ablution like this ablution of mine and then he said: He who performed ablution like this, all his previous sins would be expiated and his prayer and going towards the mosque would have an extra reward. In the tradition narrated by Ibn 'Abda (the words are):" I came to Uthman and he performed ablution."
Reference : Sahih Muslim 229
In-book reference : Book 2, Hadith 11
USC-MSA web (English) reference : Book 2, Hadith 442
(deprecated numbering scheme)