Musnad Ahmad 30:2244

'Affan narrated to us, Hammad ibn Salamah narrated to us, Muhammad ibn 'Amr narrated to us, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The Euphrates is about to uncover a mountain of gold, so people will fight over it, until nine out of every ten are killed, and one remains."