Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The Qadiyaniyya or Ahamadiyya group is perverse group that has nothing to do with Islam. It was founded by a man named Mirza Ghulam Ahmad who is considered by Muslims as an apostate. He was an agent of English colonialists, a traitor of his people. He did all he could to turn Muslims away from the true Islam. He aimed at making Muslims give up the obligation of Jihad. He did so following hints and instructions from the English who wanted to get hold of the Muslims and their land without any defense. Each one of the beliefs of this group is enough evidence that they have nothing to do with Islam.
1. They believe wrongly that Allah The Almighty fasts, prays, sleeps and wakes up and describe Allah of other attributes that are particular to creatures.
2. They believe that the Prophet Muhammad (Peace and blessing be upon him) was not the last Prophet. Mirza claims that he was a prophet and that Angel Gabriel brings him the revealed message.
3. They believe that there is no Qur'an except what was revealed to Mirza and that there is no Hadith except what he instructs his followers to do.These are only a few fof their wrong beliefs.
On the other hand, it is forbidden to pray behind any of the Qadiyani so-called Imam. If any Muslim has prayed with them, he should make up for all the prayers he has done with them.
Allah knows best.