Musnad Ahmad 26:800

Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, from al-A'mash, from Abu Wa'il, from Abdullah ibn Mas'ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I am your forerunner at the Hawd (the Cistern). Men will be snatched away from before me, and I will say: 'O Lord, my companions!' It will be said: 'Indeed, you do not know what they innovated after you.'"