Musnad Abu Ya'la al-Mawsili 3:260

Isma'il ibn Musa narrated to us, Sharik narrated to us, from 'Ammar, from Abu Salih, from 'Ali, who said: I saw the Prophet, peace and blessings of Allah be upon him, in my sleep, and I complained to him about what I had encountered from his nation in terms of crookedness and contentiousness, and I wept. He said to me, "Do not weep, O 'Ali." Then he turned, and I turned, and behold, there were two men ascending, and there were boulders with which their heads were being crushed until they were smashed, then they would return—or he said, "they would go back." He said: So I went early to 'Ali as I used to go to him every day, until when I was at the leather-workers' market, I met the people, and they said, "The Commander of the Faithful has been killed."