Musannaf Ibn Abi Shayba 38:88

Yazid ibn Harun narrated to us, from al-Awwam, from Amr ibn Murrah, from Abu Wa'il, who said: Abu Maysarah Amr ibn Sharhabil—and he was among the best of the companions of Abdullah—saw in a dream that he had been entered into Paradise. He said: I saw domes pitched, so I said: "For whom are these?" It was said: "These are for Dhu al-Kala' and Hawshab," and they were among those who were killed with Mu'awiyah on the day of Siffin. He said: I said: "So where are Ammar and his companions?" They said: "Ahead of you." I said: "How, when they killed one another?" He said: It was said: "They met Allah and found Him to be of vast forgiveness." He said: I said: "And what did the people of the river (al-Nahrawan) do?" He said: It was said: "They met [a severe] trial."