"I saw 'Uthman bin 'Affan sitting in Maqa'id. He called for water and he performed ablution, the he said: 'I saw the Messenger of Allah sitting in this place where I am sitting, performing ablution as I have done. Then he said: "Whoever performs ablution as I have done, his previous sins will be forgiven." And the Messenger of Allah said: "And do not be conceited (due to this great virtue)." (Sahih) Another chain with similar wording.  

رَأَيْتُ رَسُولَ اللَّهِ ﷺ فِي مَقْعَدِي هَذَا تَوَضَّأَ مِثْلَ وُضُوئِي هَذَا ثُمَّ قَالَ مَنْ تَوَضَّأَ مِثْلَ وُضُوئِي هَذَا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ وَقَالَ رَسُولُ اللَّهِ ﷺ وَلاَ تَغْتَرُّوا حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ حَدَّثَنَا عَبْدُ الْحَمِيدِ بْنُ حَبِيبٍ حَدَّثَنَا الأَوْزَاعِيُّ حَدَّثَنِي يَحْيَى حَدَّثَنِي مُحَمَّدُ بْنُ إِبْرَاهِيمَ حَدَّثَنِي عِيسَى بْنُ طَلْحَةَ حَدَّثَنِي حُمْرَانُ عَنْ عُثْمَانَ عَنِ النَّبِيِّ ﷺ نَحْوَهُ