Musnad Ahmad 30:1200

Al-Aswad narrated to us, he said: Kamil—meaning Abu al-Ala’—informed us, he said: I heard Abu Salih—who was a mu'adhdhin who used to call the adhan for them—say: I heard Abu Hurairah saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Seek refuge in Allah from the beginning of the seventies, and the rule of the young boys."