Private answer
Duaa after Fardh Salaat is proven.
It should be in-audable. Audible if the Imaam has to teach the Musallees, once they know then in-audible.
It is part of the etiquette of Duaa to raise the hands, thus permissible.
The Duaa should be short if there is Sunnat Salaat therafter.
The Duaa can be slightly longer if there is NO Sunnat Salaat thereafter like Fajr and Asr.
The Duaa is Mustahb.
There must delay between after the Duaa and the performance of Sunnat like reading a Kitaab for few minutes, this is not correct.
If a person does NOT make this Duaa and walks off whilst the Imaam is making this Duaa, thus person will NOT be sinful.
So classify this Duaa as Bidaat is erroneous.
Marked as spam