Musnad Ahmad 26:300

Hashim and Hassan ibn Musa narrated to us, they said: Shayban narrated to us, from Asim, from Abu Wa'il, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I am your forerunner at the Basin (al-Hawd). I will certainly dispute with some men from my companions, and I will certainly be overcome regarding them. Then it will be said to me: 'You do not know what they innovated after you.'"