IN-37
What to say while sitting in an assembly
Ibn Umar (RA) said: Allah's Messenger(SAW) used to repeat in a single sitting:
Rabbighfir lee wa tub 'alayya 'innaka 'Antat-Tawwaabul-Ghafoor.
My Lord, forgive me and accept my repentance, You are the Ever-Relenting, the All-Forgiving.
Reference: Sahih Ibn Majah 2/321. See also Al-Albani, Sahih At-Tirmithi 3/153
The Expiation of Assembly - Kaffaratul-Majlis
Subhaanaka Allaahumma wa bihamdika, 'ash-hadu 'an laa 'ilaaha 'illaa 'Anta, 'astaghfiruka wa 'atoobu 'ilayka.
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-Tirmithi and An-Nasa'i. See also Al-Albani, Sahih At-Tirmithi 3/ 153. Aishah (RA) said: "Allah's Messenger (SAW) 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
Invocation for someone who says: "May Allah forgive you"
Walaka
And you.
Reference: Ahmad 5/82, and An-Nasa'i in 'Amalul-Yawm wal-Laylah p. 218, with checking by Dr. Farooq Hamadah.
Invocation for someone who does good to you
Jazaakallaahu khayran.
May Allah reward you with good.
Reference: At-Tirmithi (no. 2035). See also Al-Albani, Sahih At-Tirmithi 2/200 and Sahihul-Jami' (no. 6244).
