ʿAmmār and the Transgressing Party

The third miracle: the Messenger of Allah (peace be upon him) saw ʿAmmār ibn Yāsir working hard, and he told him, Yā ʿAmmār, taqtuluka al-fiʾah al-bāghiyah: "O ʿAmmār, you will be killed by the aggressing party." That's all: you will be killed by the aggressing party. (Muslim 2915, 2916)

ʿAmmār lives through the time of the Messenger of Allah (peace be upon him), lives through the era of Abū Bakr, ʿUmar, ʿUthmān, and then, in the time of ʿAlī, he joins the army of ʿAlī and was killed by the army of Muʿāwiyah. So this hadith tells us that the party of ʿAlī was right, while the party of Muʿāwiyah was wrong; and this was news of al-ghayb that the Messenger of Allah (peace be upon him) was given by Allah, the Mighty and Majestic: he was told that ʿAmmār will be killed by the aggressing party. Now, both of them are Muslim parties, but one of them was right, and one of them was wrong.