Musnad Ahmad 26:160

Yazid narrated to us, Al-Awwam informed us, Abu Ishaq Al-Shaybani narrated to me, from Al-Qasim ibn Abd al-Rahman, from his father, from Abdullah, from the Prophet, peace and blessings of Allah be upon him, who said: "The millstone of Islam will rotate at the head of thirty-five, or thirty-six, or thirty-seven years. If they perish, then it is the path of those who perished. And if they remain, their religion will be established for them for seventy years."