اللَّهُمَّ اجْعَلْنِي مِنَ التَّوَّابِينَ وَاجْعَلْنِي مِنَ الْمُتَطَهِّرِينَ
Allahumma-j'alni minat-tawwabina waj'alni minal-mutatahhirin
Oh Allah, hazme de los que se arrepienten y hazme de los que se purifican.
Say after completing wudu, following the shahada
This dua asks Allah to make you among those who constantly seek forgiveness and purification.