Assalaamu alaykum. In our masjids, the imam supplicates together with the congregation (which is considered a religious innovation) after the prayer. So some brothers leave the masjid as the imam starts supplicating in order to show their dislike towards religious innovations. However, in that case, they do not complete their Thikr (remembrance of Allah) after the prayer, and they also miss the reward of remaining seated at the prayer place. So what should we do?
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If this imaam is following the opinion of a trustworthy scholar, then there is no harm on him. What should be done is to give him advice and explain the ruling of his action and the statements of scholars in this regard to him. If he heeds the advice, all praise be to Allah; otherwise, those who advised him have cleared themselves of their liability and they bear no sin if they sit and refrain from joining in this supplication as long as they believe that it is not prescribed. If they leave, there is also no harm on them.
Completing Thikr after prayer does not have to be done in the place they were sitting in; rather, they may complete it on the road.
It should be noted that such matters that some scholars permitted should not be a source of discord and dispute among the Muslims. Rather, they should kindly exchange sincere advice, clarifying the relevant evidence while maintaining affection and a friendly spirit among themselves.
Allah knows best.
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