Hadith: No Muslim person who, when an obligatory prayer becomes due, perfects its ablution, humility, and bowing except that it becomes an expiation for the sins that preceded it as long as he has not committed a major sin, and this applies to all times

عَنْ عُثْمَانَ رضي الله عنه قَالَ: سَمِعْتُ رَسُولَ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ يَقُولُ:
«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».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 228]

‘Uthmān (may Allah be pleased with him) reported that he heard the Messenger of Allah (may Allah's peace and blessings be upon him) say:
"No Muslim person who, when an obligatory prayer becomes due, perfects its ablution, humility, and bowing except that it becomes an expiation for the sins that preceded it as long as he has not committed a major sin, and this applies to all times."
Sahih/Authentic. - [Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) clarified that no Muslim who, when the time for an obligatory prayer becomes due, perfects and completes its ablution, then prays with humility in such a way that his heart and all of his body organs are devoted to Allah and heedful of His greatness, and he perfects the acts of prayer like bowing, prostration and other actions, except that this prayer will serve as an expiation for the preceding minor sins as long as he has not committed a major sin; and this merit applies to all times and every prayer.