Reference : Hisn al-Muslim 14

اللّهُـمَّ اجْعَلنـي مِنَ التَّـوّابينَ
وَاجْعَـلْني مِنَ المتَطَهّـرين.
Allāhummaj`alnī minat-tawwābīna 
waj`alnī minal-mutaṭahhirīn.

O Allah, make me among those who turn to You in repentance, 
and make me among those who are purified.

Reference:
At-Tirmidhi 1/78. See also Al-Albani, Sahih At- Tirmidhi 1/18

Chapter: Upon completing the ablution