Musnad Ahmad 430
It was narrated that Humran bin Aban said:
We were with `Uthman bin `Affan (رضي الله عنه), he called for water and did wudoo’. When he had finished his wudoo`, he smiled and said: Do you know why I smiled?  He said: The Messenger of Allah (ﷺ) did wudoo` as I just did wudoo`, then he smiled and said: “Do you know why I smiled?` we said: Allah and His Messenger know best. He said: “If a person does wudoo and completes his wudoo’, then he starts to pray and completes his prayer, he will come out of his prayer free of sin as he came out of his mother`s womb.”
حَدَّثَنَا إِسْحَاقُ بْنُ يُوسُفَ، حَدَّثَنَا عَوْفٌ الْأَعْرَابِيُّ، عَنْ مَعْبَدٍ الْجُهَنِيِّ، عَنْ حُمْرَانَ بْنِ أَبَانَ، قَالَ كُنَّا عِنْدَ عُثْمَانَ بْنِ عَفَّانَ رَضِيَ اللَّهُ عَنْهُ فَدَعَا بِمَاءٍ فَتَوَضَّأَ فَلَمَّا فَرَغَ مِنْ وُضُوئِهِ تَبَسَّمَ فَقَالَ هَلْ تَدْرُونَ مِمَّا ضَحِكْتُ قَالَ فَقَالَ تَوَضَّأَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَمَا تَوَضَّأْتُ ثُمَّ تَبَسَّمَ ثُمَّ قَالَ هَلْ تَدْرُونَ مِمَّ ضَحِكْتُ قَالَ قُلْنَا اللَّهُ وَرَسُولُهُ أَعْلَمُ قَالَ إِنَّ الْعَبْدَ إِذَا تَوَضَّأَ فَأَتَمَّ وُضُوءَهُ ثُمَّ دَخَلَ فِي صَلَاتِهِ فَأَتَمَّ صَلَاتَهُ خَرَجَ مِنْ صَلَاتِهِ كَمَا خَرَجَ مِنْ بَطْنِ أُمِّهِ مِنْ الذُّنُوبِ‏.‏