What is the evidence for the permissibility of a congregant praying ahead of the imam in cases of extreme crowding, as occurs in the Haram?

A congregant moving ahead of the Imam due to overcrowding does not invalidate the prayer. This is a view held by some scholars and is considered the most equitable and preferable opinion. Their evidence for this is that obligations are waived in the presence of an excuse. There are other scholarly opinions regarding a congregant moving ahead of the Imam: some hold that it is absolutely valid, though disliked, while others maintain that it is absolutely invalid.