Musnad Ahmad 937:149

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Sulayman, from Mundhir al-Thawri, from some of their elders, from Abu Dharr, that the Messenger of Allah (ﷺ) [saw two sheep butting each other]. And Abu Mu‘awiyah narrated to us, from al-A‘mash, from Mundhir ibn Ya‘la Abu Ya‘la, from some of his elders, from Abu Dharr, and he mentioned its meaning: That the Messenger of Allah (ﷺ) saw two sheep butting each other. He said: "O Abu Dharr, do you know why they are butting each other?" He said: "No." He said: "But Allah knows, and He will judge between them."