Musnad Ahmad 30:1381

'Affan narrated to us, Wuhaib narrated to us, 'Abdullah ibn Tawus narrated to us, from his father, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Today, an opening has been made in the barrier of Gog and Magog like this," and Wuhaib made a knot [with his fingers] representing ninety.