Reference : Hisn al-Muslim 15

سُبْحـانَكَ اللّهُـمَّ وَبِحَمدِك
أَشْهَـدُ أَنْ لا إِلهَ إِلاّ أَنْتَ 
أَسْتَغْفِرُكَ وَأَتوبُ إِلَـيْك
Subḥānaka Allāhumma wa biḥamdika, 
'ash-hadu 'an lā 'ilāha 'illā 'Anta,
'astaghfiruka wa 'atūbu 'ilayk.

Glory is to You, O Allah, and praise; 
I bear witness that there is none 
worthy of worship but You. 
I seek Your forgiveness and turn to You in 
repentance.

Reference:
An-Nasa'i, 'Amalul-Yawm wal-Laylah, p. 173. See also Al-Albani, 
'Irwa'ul-Ghalil 1/135 and 2/94.

Chapter: Upon completing the ablution