Musnad Abu Ya'la al-Mawsili 152:230

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, al-Awam bin Hawshab informed us, Abu Ishaq al-Shaybani narrated to me, from al-Qasim bin Abd al-Rahman, from his father, from Abdullah, who said: The Messenger of Allah (ﷺ) said: "The millstone of Islam will revolve at the head of thirty-five, or thirty-six, or thirty-seven. If they perish, then it is by the path of those who perished. And if they remain, their religion will remain for them for seventy years."