Prayer
مَا مِنْ مُسْلِمٍ تَحْضُرُهُ صَلَاةٌ مَكْتُوبَةٌ فَيُحْسِنُ وُضُوءَهَا وَخُشُوعَهَا وَرُكُوعَهَا، إِلَّا كَانَتْ كَفَّارَةً لِمَا قَبْلَهَا مِنَ الذُّنُوبِ مَا لَمْ يُؤْتِ كَبِيرَةً وَذَلِكَ الدَّهْرَ كُلَّهُ
Translation
There is no Muslim who performs an obligatory prayer and perfects its ablution, humility, and bowing, except that it will be an expiation for his previous sins as long as he does not commit a major sin, and this applies for all time.
Narrator
Uthman ibn Affan
Source
Sahih Muslim
Muslim 228