I was with Uthman, and he called for ablution water and said: I heard Messenger of Allah ﷺ say: When the time for a prescribed prayer comes, if any Muslim performs ablution well and offers his prayer with humility and bowing, it will be an expiation for his past sins, so long as he has not committed a major sin; and this applies for all times.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ مَا مِنِ امْرِئٍ مُسْلِمٍ تَحْضُرُهُ صَلاَةٌ مَكْتُوبَةٌ فَيُحْسِنُ وُضُوءَهَا وَخُشُوعَهَا وَرُكُوعَهَا إِلاَّ كَانَتْ كَفَّارَةً لِمَا قَبْلَهَا مِنَ الذُّنُوبِ مَا لَمْ يُؤْتِ كَبِيرَةً وَذَلِكَ الدَّهْرَ كُلَّهُ  

I was with Uthman, and he called for ablution water and said: I heard Messenger of Allah ﷺ say: When the time for a prescribed prayer comes, if any Muslim performs ablution well and offers his prayer with humility and bowing, it will be an expiation for his past sins, so long as he has not committed a major sin; and this applies for all times. (Using translation from Muslim 228)  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ «مَا مِنِ امْرِئٍ مُسْلِمٍ تَحْضُرُهُ الصَّلَاةُ الْمَكْتُوبَةُ فَيُحْسِنُ وُضُوءَهَا وَرُكُوعَهَا وَخُشُوعَهَا إِلَّا كَانَتْ كَفَّارَةً لِمَا قَبْلَهَا مِنَ الذُّنُوبِ مَا لَمْ يَأْتِ كَبِيرَةً وَذَلِكَ الدَّهْرَ كُلَّهُ»  

[Machine] I witnessed the Messenger of Allah ﷺ say, "There is no Muslim who attends a prescribed prayer and performs its ablution, bowing, and prostration well, except that it will be an expiation for what has passed of sins, as long as they were not major sins, and this applies for their entire lifetime."  

شَهِدْتُ رَسُولَ اللهِ ﷺ يَقُولُ مَا مِنْ مُسْلِمٍ تَحْضُرُهُ صَلَاةٌ مَكْتُوبَةٌ فَيُحْسِنُ وُضُوءَهَا وَرُكُوعَهَا وَسُجُودَهَا إِلَّا كَانَتْ لَهُ كَفَّارَةً لِمَا مَضَى مِنَ الذُّنُوبِ مَا لَمْ يَأْتِ كَبِيرَةً وَهَذَا الدَّهْرَ كُلَّهُ  

"مَا مِن امْرِئ مُسْلِمٍ تَحْضُرُه صَلاةٌ مَكْتُوبَةٌ فَيُحْسِنُ وُضُوءَهَا وَخُشُوعَهَا وَرُكُوعَها إلا كَانت كفَّارَةً لمَّا قَبْلَهَا من الذُّنُوب مَا لَمْ تُؤْتَ كَبيرَةٌ، وَذَلك الدِّهْرَ كُلَّه".