Musnad Ahmad 32:1881

Affan narrated to us, Hammad bin Salamah narrated to us, from Ali bin Zaid, from Anas, that the Messenger of Allah (ﷺ) said: "I saw in what the sleeper sees, as if I were riding behind a ram, and as if the tip of my sword had broken. So I interpreted it that I would kill the commander of the battalion..."