How do we know about "how to perform salah"?. The answer must be: from our Islamic teachers. And how our teachers knew about it?. Must be from their teachers. And their teachers knew from previous teachers.
Yes, there is Sanad in this matter. And if we trace this Sanad we would arrive to Grand Fuqaha in the past, and finally we arrive to Rasulullah Shallallahu Alayhi Wasalam.
So, it's better if we do not easy to say our brothers way in perform salah is against what Rasulullah Shallallahu Alayhi Wasalam had taught Muslims, just because how they perform salah is not really same with us. Yes, there are little diversities in Fiqh of salah, but we don't need to 'blow up' these Fururiyah diversities into a tension between us and our Muslim brothers.