كتاب الطهارة
2
The Book of Purification
(6)
Chapter: Adh-Dhikr (the remembrance) which is recommended following wudu’
(6)
باب الذِّكْرِ الْمُسْتَحَبِّ عَقِبَ الْوُضُوءِ ‏‏
Sahih Muslim 234a

'Uqba b. 'Amir reported:

We were entrusted with the task of tending the camels. On my turn when I came back in the evening after grazing them in the pastures, I found Allah's Messenger (ﷺ) stand and address the people. I heard these words of his: If any Muslim performs ablution well, then stands and prays two rak'ahs setting about them with his heart as well as his face, Paradise would be guaranteed to him. I said: What a fine thing is this! And a narrator who was before me said: The first was better than even this. When I cast a glance, I saw that it was 'Umar who said: I see that you have just come and observed: If anyone amongst you performs the ablution, and then completes the ablution well and then says: I testify that there is no god but Allah and that Muhammad is the servant of Allah and His Messenger, the eight gates of Paradise would be opened for him and he may enter by whichever of them he wishes.
حَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمِ بْنِ مَيْمُونٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، حَدَّثَنَا مُعَاوِيَةُ بْنُ صَالِحٍ، عَنْ رَبِيعَةَ، - يَعْنِي ابْنَ يَزِيدَ - عَنْ أَبِي إِدْرِيسَ الْخَوْلاَنِيِّ، عَنْ عُقْبَةَ بْنِ عَامِرٍ، ح

وَحَدَّثَنِي أَبُو عُثْمَانَ، عَنْ جُبَيْرِ بْنِ نُفَيْرٍ، عَنْ عُقْبَةَ بْنِ عَامِرٍ، قَالَ كَانَتْ عَلَيْنَا رِعَايَةُ الإِبِلِ فَجَاءَتْ نَوْبَتِي فَرَوَّحْتُهَا بِعَشِيٍّ فَأَدْرَكْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم قَائِمًا يُحَدِّثُ النَّاسَ فَأَدْرَكْتُ مِنْ قَوْلِهِ ‏"‏ مَا مِنْ مُسْلِمٍ يَتَوَضَّأُ فَيُحْسِنُ وُضُوءَهُ ثُمَّ يَقُومُ فَيُصَلِّي رَكْعَتَيْنِ مُقْبِلٌ عَلَيْهِمَا بِقَلْبِهِ وَوَجْهِهِ إِلاَّ وَجَبَتْ لَهُ الْجَنَّةُ ‏"‏ ‏.‏ قَالَ فَقُلْتُ مَا أَجْوَدَ هَذِهِ ‏.‏ فَإِذَا قَائِلٌ بَيْنَ يَدَىَّ يَقُولُ الَّتِي قَبْلَهَا أَجْوَدُ ‏.‏ فَنَظَرْتُ فَإِذَا عُمَرُ قَالَ إِنِّي قَدْ رَأَيْتُكَ جِئْتَ آنِفًا قَالَ ‏"‏ مَا مِنْكُمْ مِنْ أَحَدٍ يَتَوَضَّأُ فَيُبْلِغُ - أَوْ فَيُسْبِغُ - الْوُضُوءَ ثُمَّ يَقُولُ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا عَبْدُ اللَّهِ وَرَسُولُهُ إِلاَّ فُتِحَتْ لَهُ أَبْوَابُ الْجَنَّةِ الثَّمَانِيَةُ يَدْخُلُ مِنْ أَيِّهَا شَاءَ ‏"‏ ‏.‏

Reference : Sahih Muslim 234a
In-book reference : Book 2, Hadith 20
USC-MSA web (English) reference : Book 2, Hadith 451
  (deprecated numbering scheme)