Mishkat al-Masabih
Purification — Hadith #268
وَعَنْ عُثْمَانَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَا مِنَ امْرِئٍ مُسْلِمٍ تَحْضُرُهُ صَلَاةٌ مَكْتُوبَةٌ فَيُحْسِنُ وُضُوءَهَا وَخُشُوعَهَا وَرُكُوعَهَا إِلَّا كَانَتْ كَفَّارَةً لِمَا قَبْلَهَا مِنَ الذُّنُوبِ مَا لَمْ يُؤْتِ كَبِيرَةً وَذَلِكَ الدَّهْرَ كُلَّهُ» . رَوَاهُ مُسلم
‘Uthman reported God’s messenger as saying, “When the time for a prescribed prayer comes, if any Muslim observes proper ablution, humility and bowing, it will be an expiation for his past sins, so long as he has not committed a major sin; and that means for all time.”
Muslim transmitted it.