Members of the congregation saying “Nashhadu (we bear witness) and “Haqqan (indeed)” when the imam praises Allaah
Question

                Sometimes we hear some members of the congregation saying, Ameen, Nashhadu (we bear witness), Subhaanaka (Glory be to You) etc, during Du’aa’ al-Qunoot. If that is narrated in any report, please explain to us when it should be said.
Praise be to Allah.
    The Sunnah is that when the imam recites Du’aa’ al-Qunoot 
    during the prayer, the members of the congregation should say Ameen to his 
    du’aa’. But when the imam is praising Allaah, it is better for them to 
    remain silent, because the imam’s praise is sufficient. As for some of the 
    members of the congregation saying, Nashhadu (we bear witness) or “Ya 
    Allaah” and so on, there is no basis for this. 

    Shaykh Bakr Abu Zayd said: 
Some mistakes and 
    innovations are sometimes done during the du’aa’ and dhikr, such as the 
    following: …

     45. Answering the dhikr during the qunoot of the imam by 
    saying “haqqan (indeed) or “sadaqta (you have spoken the truth)” or “sadaqan 
    wa ‘adalan (that is right and true)” or “ashhadu (I bear witness)” or “haqq 
    (truth)” and so on. There is no basis for saying any of these things. 

    Tasheeh al-Du’aa’, p. 419-423.