DOA KAFFARATUL MAJLIS ..

The phrase

 "Subhanakallahumma wabihamdika wa astaghfiruka"

 is part of a well-known Islamic supplication known as 
Kaffarah al-Majlis 
(the Expiation of the Gathering). 

It is recited to glorify Allah and seek His forgiveness, particularly when concluding a meeting, conversation, or social gathering. 

The complete, standard version of this supplication is recorded in collections such as

  Jami'at-Tirmidhi:
No 3433 

 سُبْحَانَكَ اللَّهُمَّ وَبِحَمْدِكَ، أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا أَنْتَ، أَسْتَغْفِرُكَ وَأَتُوبُ إِلَيْكَ

 "Glory be to You, O Allah, and all praise is due to You. 

When to Recite It

According to authentic narrations, this dua is recommended in several situations: 

Ending a Gathering: 

After Wudu (Ablution):
 
Ending Ruku and Sujood in Prayer:

 Subhanakallahumma wa bihamdika, ash-hadu an la ilaha illa Anta, astaghfiruka wa atubu ilayk.

I bear witness that there is no deity worthy of worship except You.
 I seek Your forgiveness and 
I repent to You." 

The Prophet Muhammad
 (peace be upon him)
 taught that reciting this before standing up from a gathering expiates any idle talk, mistakes, or minor sins that occurred during the sitting. 

 Narrations recorded by 

Imam an-Nasa'i and others also recommend reciting these words after completing ablution.

 A shorter variation

 (Subhanakallahumma Rabbana wa bihamdika, Allahummaghfir li) 
was frequently recited by the Prophet (peace be upon him) 
while bowing and prostrating in Salah.

WALLAHU A'LAM ..