Musnad Ahmad 30:3502

Hassan narrated to us, Shaiban narrated to us, from Qatadah, from Abu Rafi’, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Gog and Magog..." and he mentioned its meaning, except that he said: "When their time reaches its end, and Allah the Almighty and Majestic wills to send them forth upon the people."