Sahih Muslim Book - 2, Hadith - 543

حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ وَحَجَّاجُ بْنُ الشَّاعِرِ كِلَاهُمَا عَنْ أَبِي الْوَلِيدِ قَالَ عَبْدٌ حَدَّثَنِي أَبُو الْوَلِيدِ حَدَّثَنَا إِسْحَاقُ بْنُ سَعِيدِ بْنِ عَمْرِو بْنِ سَعِيدِ بْنِ الْعَاصِ حَدَّثَنِي أَبِي عَنْ أَبِيهِ قَالَ كُنْتُ عِنْدَ عُثْمَانَ فَدَعَا بِطَهُورٍ فَقَالَ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ مَا مِنْ امْرِئٍ مُسْلِمٍ تَحْضُرُهُ صَلَاةٌ مَكْتُوبَةٌ فَيُحْسِنُ وُضُوءَهَا وَخُشُوعَهَا وَرُكُوعَهَا إِلَّا كَانَتْ كَفَّارَةً لِمَا قَبْلَهَا مِنْ الذُّنُوبِ مَا لَمْ يُؤْتِ كَبِيرَةً وَذَلِكَ الدَّهْرَ كُلَّهُ.

Ishaq bin Sa'eed bin 'Amr bin Sa'eed bin Al-'As narrated: "My father told me that his father said: 'I was with 'Uthman and he called for water for purification. He said: "I heard the Messenger of Allah (s.a.w) say: 'There is no Muslim man who, when the time for a Salat Maktubah (prescribed prayer) comes, performs Wudu' well, focuses with proper humility in his prayer and bows properly, but it will be an expiation for the sins that came before it, so long as he did not commit any major sin, and that applies for all time."'