سُبْحَانَكَ اَللَّهُمَّ وَبِحَمْدِكَ، أَشْهَدُ أَنْ لَا إِلَهَ إِلاَّ أَنْتَ، أَسْتَغْفِرُكَ وَأَتُوْبُ إِلَيْكَ

Subhaanaka Allaahumma wa bihamdika, 'ash-hadu 'an laa 'ilaaha 'illaa 'Anta, 'astaghfiruka wa 'atoobu '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. Abu Sayed (RA) said, Rasulullah (ﷺ) said, if someone says this dua after ablution, it will be written in a letter and sealed (under the throne). That seal will not be broken before the Resurrection.

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