Musnad Ahmad 26:492

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Shaqiq, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I am your forerunner at the Hawd (the Cistern). I will certainly dispute with some people, and then I will be overcome regarding them. I will say: 'O my Lord, my companions!' It will be said: 'Indeed, you do not know what they innovated after you.'"