Praise be to Allah.
With regard to dhikr recited in unison, the Standing Committee was asked about du`a and dhikr recited in unison, and they answered:
The basic principle concerning dhikr and acts of worship is that there is no room for adding or subtracting anything. Allah should only be worshipped in the ways that He has prescribed; this applies whether it is something that can be done at any time or something that is limited to a specific time and how it is to be done and how many times it is to be done. With regard to dhikrs and du`as that Allah has prescribed and all kinds of worship that are not limited with regard to time and numbers, location or a particular way in which they are to be done, it is not permissible for us to adhere to a particular way, time or number; rather we are to do these acts of worship without restrictions as it was prescribed.
In matters where it isproven in the words or deeds of the Prophet (peace and blessings of Allah be upon him) that an action is to be done at a certain time or a certain number of times or in a certain place or in a certain manner, then we should worship Allah in accordance with what has been prescribed. It has not been proven from the words or deeds or approval of the Prophet (peace and blessings of Allah be upon him) that he recited du`a in unison with others following the prayers, or after reading Quran, or following each lesson, whether that took the form of the imaam reciting du`a and the congregation saying Ameen to his du`a, or whether they all recited du`a together in unison. That was not known at the time of the Rightly-Guided Khaleefahs or any of the Sahaabah (may Allah be pleased with them). Whoever adheres to the practice of reciting du`a in unison following the prayers or after reading Quran or after every lesson has innovated something and introduced into the religion something which is not a part of it. It was narrated that the Prophet (peace and blessings of Allah be upon him) said: “Whoever innovates something in this matter of ours (i.e. Islam) that is not part of it will have it rejected.” And he said: “Whoever does something that is not in accordance with this matter of ours (Islam) will have it rejected.”
If it were prescribed to adhere to a certain way of doing it, the Prophet (peace and blessings of Allah be upon him) and his successors (khulafa’) after him would have adhered to that. We have stated above that no such thing was proven from him or from his companions (may Allah be pleased with them). All goodness is to be found in following the guidance of the Prophet (peace and blessings of Allah be upon him) and the guidance of the Rightly-Guided Khulafa’ (may Allah be pleased with them); all evil is to be found in going against their guidance and following the innovated matters against which the Prophet (peace and blessings of Allah be upon him) warned us by saying, “Beware of newly-innovated matters, for every innovation is misguidance.” May Allah bless our Prophet Muhammad and his family and companions, and grant them peace.
Fatawa Islamiyyah, 4/178