Musnad Ahmad 26:765

Yazid narrated to us, [he said] 'Awam ibn Hawshab informed us, he said: Abu Ishaq al-Shaybani narrated to me, from al-Qasim ibn 'Abd al-Rahman, from his father, from 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The millstone of Islam will revolve 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 remain for them for seventy years."