Musnad Ahmad 26:211

Hajjaj narrated to us, Sufyan narrated to us, Mansur narrated to us, from Rib'i, from al-Bara' ibn Najiyah al-Kahili, from Abdullah ibn Mas'ud, who said: The Messenger of Allah (ﷺ) said: "Indeed, the millstone of Islam will revolve for thirty-five, or thirty-six, or thirty-seven years. If they perish, then it is the path of those who perished. And if their religion stands firm for them, it will stand firm for them for seventy years." He said: Umar said: "O Messenger of Allah, is it regarding what has passed or what remains?" He said: "Rather, it is regarding what remains."