Reference : Hisn al-Muslim 196

سُبْحـانَكَ اللّهُـمَّ وَبِحَمدِك، 
أَشْهَـدُ أَنْ لا إِلهَ إِلاّ أَنْتَ 
أَسْتَغْفِرُكَ وَأَتوبُ إِلَـيْك
Subhānaka Allāhumma wa biḥamdik, 
ash-hadu an lā ilāha illā ant,
astaghfiruka wa atību ilayk.


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

Reference:
Abu Dawud, Ibn Majah, At-Tirmidhi and An-Nasa'i. See also Al-Albani, Sahih 
At-Tirmidhi 3/ 153. Aishah (RA) said: "Allah's Messenger (ﷺ) did not sit 
in a gathering, and did not recite the Qur'an, and did not perform any 
prayer without concluding by saying ... (then she quoted the above)." This 
was reported by An-Nasa'i in 'Amalul-Yawm wal-Laylah (no.308), and Dr. 
Farooq Hamadah graded it authentic in his checking of the same book, p. 
273. See also Ahmad 6/77

Chapter: For the expiation of sins, said at the conclusion of a sitting or gathering