This is a well proven Hadith among Sunni fellows, Imam Tirmidhi graded as Sahih hadith, Bukhari also wrote that the Holy Prophet (pbuh) said a rebellious group will kill Ammar, Ammar will invite them towards heaven but the group will invite Ammar bin Yasir towards hell. We know that Ammar bim Yasir (as) got killed fighting against Moawiya's army.

 English Translation:     

Umme Salma narrated:

I do not forget the day of KHANDAQ when prophet gave the tiles while there was dust on his hairs and said “Oh Allah, the life after the death is better. Forgive the Muhajirin and Ansar. Ammar bin Yassar came there”. The Prophet said “Oh Ammar, a group of rebellious group will martyr you.”