Musnad Ahmad 26:263

Aswad ibn Amir narrated to us, Abu Bakr informed 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 contend with some men, and I will be overcome regarding them. I will say: 'O Lord, my companions!' It will be said: 'You do not know what they innovated after you.'"