Musnad Abu Ya'la al-Mawsili 152:100

Abu Khaythama narrated to us, Jarir narrated to us, from al-A'mash, from Abu Wa'il, who said: Abdullah said: The Messenger of Allah (ﷺ) said: "I am your forerunner at the Hawd (the Cistern). I will certainly dispute with some men from among you, and then I will certainly be overcome regarding them. It will be said: 'Indeed, you do not know what they innovated after you.'"