This is a prediction of Holy Prophet(pbuh) by revelation that two groups of Muslims will fought with each other.
Imam Ali's group and Moawiya bin Abu Sufyan group, another group raised called Kharijities in Islamic history. We know that they were killed by Imam Ali's army. It proves that Imam Ali's group was right and Moawiya's group was wrong. But our Sunni fellows have hesitation to say about Moawiya that he was wrong.
Please read the English translation
Abbi Saeed Khudri narrated:
The holy Prophet said “One group will be raise among the people while two groups will be fighting. That group will be tested for their killing (khawarij) which will be on truth.”
Part - B