Is it valid for a group of young men to perform one of the prayers in congregation in a mosque after the imam has finished his prayer without his permission?

The questioner : All right, in Egypt, my Shaykh, few people hear about combining prayers, especially in the case of rain falling. So, is it permissible for a person or a group of young men who know this ruling to perform a combined or shortened prayer after the congregation has ended in the same mosque?

The Shaykh : No, except with the imam's permission. The imam is the one who combines; they cannot contradict him.