It is not 'a slang'. 'Slang' is uncountable. And 'I beg you to pray' is not slang. Slang is generally considered to be inferior in some way to standard language, so we would not use it for the formulaic expressions used by religious groups. IF 'I beg you to pray' were used, we would probably refer to it as a formulaic expression. It's pointless to say any more about that formulaic expression, because we have no evidence that it is used.
For example, if we assume that ONLY English-speaking Muslims use "I beg you to pray" (I am not sure if this English equivalent is used by them, but this is a direct translation from Persian), can we claim that this is a slang? or is it an expression of that specific sub-group?, and why is it so?