Through which Hadees, the collective Dua after the Jama'th Namaz is established? Muthi Rasheed Sahib (of Dharb e Mu'min) has written in his book "Namaz ke bad Dua" that it is Bid'ath and he establishes that it is a Bid'ath in all the four Imams.
Making Dua by Imam, Muqtadi (Follower) or the individual after the conclusion of Fardh Salath, and raising hands while making Dua , which are Sunnath e Musthahabba (desirable Sunnath) are established through the Ahadees of Nabi Sallallahu Alaihi wa Sallam and commentary of Fiqh(Islamic jurisprudence). Hence if the Imam and the Muqthadi both practise this Sunnath it would subsequently become a collective action which is permissible. However making Dua silently is excellent since it is encouraged in Qur’an and Sunnath. Even it is permissible for the sake of teaching if the Imam makes Dua loudly and the followers say Ameen. And there is no harm in it according to Shari’a since, making Dua after the Salath; raising hands in Dua; saying Ameen; and, at the end of Dua wiping the face with both hands are all established through blessed Ahadees. Therefore to consider it as wrong and say it as an evil innovation (Bid’ath e Sayyi’a) is not correct.
However, the present day practice of collective Dua where Imam and the followers both make Dua together; together they begin, with Imam/Muazzin saying loudly the opening words such as ALHAMDU LILLAHI etc. and together they finish, due to which, the Muqtadi is supposed to wait for Imam’s Dua and, is unable to finish his Dua, and in case he finishes his Dua before the Dua of Imam it is considered among the people as an improper act. In certain places, those who do not say Ameen or other words in reply to the audible Dua of Imam in a raised voice are looked down upon, in some other places the Muqtadi, after completing his Salath is supposed to be seated, awaiting the conclusion of the Dua of Imam. These activities have no least connection what so ever with the pristine Shari’a and neither any proof is found for such actions in the golden era which is famous for (encompassing) all the goodness. Therefore all these scenarios are invented and (Bid’ath) innovations, whereas Mufthi Rasheed Ahmed Sahib Rahimahullah too declares these particular scenarios as Bid’ath. And it is necessary to refrain from them.
Summary is that, the collective Dua of Imam and Muqtadi is a resultant act and not the purpose. Therefore to try to make it longer than the main Dua and to consider it as essential is not correct, rather, Imam too has the choice to make Dua as much as he wishes and the Muqtadi too has the choice. In this Dua they are not dependent upon each other. As such if Muqtadi wishes he may make a brief Dua and go, or finish alongwith Imam’s Dua or make lengthy Dua after the completion of Imam’s Dua. All these mehods are permissible and adopting any of this method would validate the Sunnat e Mustahabba act after the Fardh Salath.