Dua when finishing wudhu

Ash-hadu allâ ilâha illallãhu wahdahü lâ sharika lahü wa ash-hadu anna muhammadan abduhü wa rasüluh.

I bear witness that there is no Lord except Allah (SWT). He is one and has no partner and I bear witness that Muhammad is His Servant and Messenger. (Muslim)

Note: Whoever recites this dua, the eight doors of Paradise will be opened for him.

Then Recite;

Allãhummaj-alni minattawwâbina wa- alni minal mutatahhirin.

O Allah, make me from amongst the repenters and among those who keep very clean. (Tirmidhi)