The Dua After Wudu
أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ، اللَّهُمَّ اجْعَلْنِي مِنَ التَّوَّابِينَ وَاجْعَلْنِي مِنَ الْمُتَطَهِّرِينَ
Ash-hadu an la ilaha illallahu wahdahu la sharika lah, wa ash-hadu anna Muhammadan 'abduhu wa rasuluh. Allahummaj'alni minat-tawwabina waj'alni minal-mutatahhirin.
I bear witness that there is no god but Allah alone, with no partner, and I bear witness that Muhammad is His servant and His Messenger. O Allah, make me one of those who turn back to You often, and make me one of those who keep themselves pure.
What it means
Wudu washes the limbs, and this dua asks for the matching cleanliness of the heart. The two things requested go together: to keep returning after every slip, and to keep yourself clean in the first place.
When and how
Say it once immediately after finishing wudu, standing and facing the qiblah if that is easy.
Its virtue
The Prophet ﷺ said that whoever performs wudu well and then says the testimony of faith will have the eight gates of Paradise opened for him to enter through whichever he wishes, reported in Sahih Muslim 234. The addition asking to be among those who turn back and those who purify themselves is reported in Jami at-Tirmidhi 55.
Source
Muslim 234; At-Tirmidhi 55sahih