Musnad Ahmad 484
It was narrated that Humran said. `Uthman (رضي الله عنه) did ghusl every day since he became Muslim. I  brought water for him one day to do wudoo’ for prayer. After he did wudoo’, he said:
I wanted to tell you a hadeeth that I heard from the  Messenger of Allah (ﷺ). Then I  decided not to tell it to you, al-Hakam bin al-`As said: O Ameer  al-Mu`mineen, if it is enjoining us  to do good we will do it and if it is warning us of a bad thing we will avoid it. He said: I will tell it to you  the Messenger of Allah (ﷺ)  did  wudoo in this manner, then he said: `Whoever does wudoo’ in this manner and does it well, then goes and prays, bowing and prostrating properly, it (the prayer) will  expiate his sins between that - prayer and the next, unless he does  or something serious` i.e., major sin.
حَدَّثَنَا عَفَّانُ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ عَاصِمٍ، عَنِ الْمُسَيَّبِ، عَنْ مُوسَى بْنِ طَلْحَةَ، عَنْ حُمْرَانَ، قَالَ كَانَ عُثْمَانُ رَضِيَ اللَّهُ عَنْهُ يَغْتَسِلُ كُلَّ يَوْمٍ مَرَّةً مِنْ مُنْذُ أَسْلَمَ فَوَضَعْتُ وَضُوءًا لَهُ ذَاتَ يَوْمٍ لِلصَّلَاةِ فَلَمَّا تَوَضَّأَ قَالَ إِنِّي أَرَدْتُ أَنْ أُحَدِّثَكُمْ بِحَدِيثٍ سَمِعْتُهُ مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ثُمَّ قَالَ بَدَا لِي أَنْ لَا أُحَدِّثَكُمُوهُ فَقَالَ الْحَكَمُ بْنُ أَبِي الْعَاصِ يَا أَمِيرَ الْمُؤْمِنِينَ إِنْ كَانَ خَيْرًا فَنَأْخُذُ بِهِ أَوْ شَرًّا فَنَتَّقِيهِ قَالَ فَقَالَ فَإِنِّي مُحَدِّثُكُمْ بِهِ تَوَضَّأَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ هَذَا الْوُضُوءَ ثُمَّ قَالَ مَنْ تَوَضَّأَ هَذَا الْوُضُوءَ فَأَحْسَنَ الْوُضُوءَ ثُمَّ قَامَ إِلَى الصَّلَاةِ فَأَتَمَّ رُكُوعَهَا وَسُجُودَهَا كَفَّرَتْ عَنْهُ مَا بَيْنَهَا وَبَيْنَ الصَّلَاةِ الْأُخْرَى مَا لَمْ يُصِبْ مَقْتَلَةً يَعْنِي كَبِيرَةً‏.‏