Musnad Ahmad 486
It was narrated from ‘lkrimah bin Khalid:
a man from Madinah told me that the mu`adhdhin gave the call to `Asr prayer and ‘Uthman (رضي الله عنه) called for water and purified himself, then he said: I heard the Messenger of Allah (ﷺ) say: 
`Whoever purifies himself as he has been enjoined and prays as he has been enjoined, his sins will be expiated.” Then he called four of the Companions of the Messenger of Allah (ﷺ)  to testify to that, and they testified that the Prophet (ﷺ)  had said that.
قَالَ حَدَّثَنَا عَفَّانُ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ إِبْرَاهِيمَ بْنِ الْمُهَاجِرِ، عَنْ عِكْرِمَةَ بْنِ خَالِدٍ، حَدَّثَنِي رَجُلٌ، مِنْ أَهْلِ الْمَدِينَةِ أَنَّ الْمُؤَذِّنَ، أَذَّنَ لِصَلَاةِ الْعَصْرِ قَالَ فَدَعَا عُثْمَانُ رَضِيَ اللَّهُ عَنْهُ بِطَهُورٍ فَتَطَهَّرَ قَالَ ثُمَّ قَالَ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ مَنْ تَطَهَّرَ كَمَا أُمِرَ وَصَلَّى كَمَا أُمِرَ كُفِّرَتْ عَنْهُ ذُنُوبُهُ فَاسْتَشْهَدَ عَلَى ذَلِكَ أَرْبَعَةً مِنْ أَصْحَابِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ فَشَهِدُوا لَهُ بِذَلِكَ عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ‏.‏