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.