Du’aa for atonement of the Gathering
Subhaanaka Allaahumma wa bihamdika,
‘ash-hadu ‘an laa ‘ilaaha ‘illaa ‘Anta, ‘astaghfiruka wa ‘atoobu ‘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 
 Abu Dawood, Ibn Maajah, At-Tirmidhi and An-Nasaai. See also Al-Albanee, Sahih At-Tirmidhi 3/153.
Aisha (radhi Allaahu anha) said:
“Allaah’s messenger (peace be upon him) 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 wa reported by An-Nasaa’i in ‘Amalul-Yawm wal-Laylah (no.308). See also Ahmad 6/77
It is also one of the supplications to be said after completing ablution (wudhu). 
 An-Nasaa’i, ‘Amalul-Yawm wal-Laylah, p. 173. See also Al-Albanee, ‘Irwa ‘ul-Ghalil 1/135 and 2/94. ( see. Hisn al Muslim, Dar-us-salam)
Source : Fortress of The Muslim – Hisn Al Muslim – Dar-us-Salam publications
The following is taken from – the concluding statement from ” The Prophet’s Prayer Described” – by Shaykh Muhammad Naasiruddeen al-Albaanee
«Glorified be Allaah, and Praised. Glorified be You, O Allaah, and Praised. I bear witness that there is no true god except You. I seek forgiveness from You and repent to You.O Allaah! send prayers on Muhammad, and on the family of Muhammad, and send blessings on Muhammad, and on the family of Muhammad, as You sent prayers on Ibraaheem and the family of Ibraaheem; You are indeed Worthy of Praise, Full of Glory»
 The first supplication is the fullest form of the du`aa’ known as kaffaarah al-majlis (expiation of the gathering); “he who says it in a gathering of Remembrance (of Allaah), it will be like a seal to stamp it with, and he who says it in a gathering of vain talk, it will be an expiation for it” – authentically related by al-Haakim and at-Tabaraanee. The second supplication is, of course, from the Sunnah of sending peace and mercy on the Messenger. These two supplications are thus the best way of implementing the following Islamic guideline: “No people sit in a gathering in which they do not mention Allaah, nor send prayers on the Prophet, without it being a source of regret for them; if Allaah wishes, He will punish them, or if He wishes, he will forgive them” – authentically related by at-Tirmidhee, al-Haakim and Ahmad. See Shaykh Albaanee’s Silsilah al-Ahaadeeth as-Saheehah (74-81) for details.