Is it permissible to use a cellphone during a mustahab salat in order to read it the right way if we have not memorized the instructions? Like salat-e-witr for example. Also, is it permissible to use a cellphone to read a specific dua during qunoot?

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This is no different to holding a book. You are reading the same thing, only the medium is different. So it’s ok for what you have asked about.