Hadith: If anyone performs ablution like this ablution of mine and offers two Rak'ahs during which he does not think of anything else, Allah will forgive his past sins

عَنْ ‌حُمْرَانَ مَوْلَى عُثْمَانَ بْنِ عَفَّانَ رضي الله عنه:
أَنَّهُ رَأَى عُثْمَانَ بْنَ عَفَّانَ دَعَا بِوَضُوءٍ، فَأَفْرَغَ عَلَى يَدَيْهِ مِنْ إِنَائِهِ، فَغَسَلَهُمَا ثَلَاثَ مَرَّاتٍ، ثُمَّ أَدْخَلَ يَمِينَهُ فِي الْوَضُوءِ، ثُمَّ تَمَضْمَضَ وَاسْتَنْشَقَ وَاسْتَنْثَرَ، ثُمَّ غَسَلَ وَجْهَهُ ثَلَاثًا، وَيَدَيْهِ إِلَى الْمِرْفَقَيْنِ ثَلَاثًا، ثُمَّ مَسَحَ بِرَأْسِهِ، ثُمَّ غَسَلَ كُلَّ رِجْلٍ ثَلَاثًا، ثُمَّ قَالَ: رَأَيْتُ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ يَتَوَضَّأُ نَحْوَ وُضُوئِي هَذَا، وَقَالَ: «مَنْ تَوَضَّأَ نَحْوَ وُضُوئِي هَذَا ثُمَّ صَلَّى رَكْعَتَيْنِ لَا يُحَدِّثُ فِيهِمَا نَفْسَهُ غَفَرَ اللهُ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 164]

Humrān, the freed slave of 'Uthmān ibn' Affān, reported that he saw 'Uthmān ibn' Affān asking (for water) to perform ablution, and he poured water from his vessel over his hands and washed them thrice and then put his right hand in the water and rinsed his mouth and washed his nose by putting water in it and then blowing it out. Then, he washed his face thrice and his forearms up to the elbows thrice. Then, he passed his wet hands over his head and washed each foot thrice. After that, he said: "I saw the Prophet (may Allah's peace and blessings be upon him) perform ablution like this ablution of mine, and he said: 'If anyone performs ablution like this ablution of mine and offers two Rak'ahs during which he does not think of anything else, Allah will forgive his past sins.'"
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 164]

Explanation

'Uthmān ibn' Affān (may Allah be pleased with him) practically taught the Prophet's manner of ablution to make it clearer. He asked for water in a vessel and poured water on his hands three times. After that, he entered his right hand into the vessel, took water with it, put it in his mouth and let it out. Then, he sniffed water into his nose and then blew it out. Then, he washed his face three times and then washed his forearms up to the elbows three times. Then, he passed his wet hand over his head once. Then, he washed his feet along with the ankles three times.
When he (may Allah be pleased with him) finished, he informed them that he saw the Prophet (may Allah's peace and blessings be upon him) perform ablution like that ablution and that he (may Allah's peace and blessings be upon him) gave them glad tidings that whoever performs ablution like his ablution and offers two Rak'ahs, with presence of mind and humility before his Almighty Lord, Allah will recompense him for this complete ablution and this sincere prayer by forgiving his past sins.