Musnad Ahmad 26:316

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: "And I am your forerunner at the Basin (al-Hawd). And I will certainly contend for some men, and I will be overcome regarding them. So I will say: 'O Lord, my companions!' And He will say: 'Indeed, you do not know what they innovated after you.'"