Re: sunni's praying in a shia mosque..
^ U cannot take any aera ghera as Imam, there is a Hadeeth about whp should be preferred for Imamat.
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for example all shias do hajj/umra where they pray behind a wahhabi imam, who would be the person they have the most disagreement with
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They do stand in the line, but there intention(Niyyat) is to pray alone(Furaada).