Reference: al-Bid’u wal-Muhdathaat wa maa laa Asla lahu – page 317
Fataawa at-Ta’ziyyah – Page 40
Question: What is the ruling regarding the congregational supplication at the graveside, whereby one of them supplicates and the rest say “aameen”?
Response: This is not from the Sunnah of the Messenger ﷺ, nor the Rightly Guided Khulafaa. Rather, the Messenger ﷺ used to guide them to seek forgiveness for the deceased and ask (Allaah) to make him firm [in answering the questions of the two angels] for him, and all this by himself [individually] and not in congregation.
