Musnad Ahmad 30:3722

Yahya ibn Ishaq narrated to us, Wahib informed me, Ibn Tawus informed me, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "An opening has been made in the barrier of Gog and Magog like this," and he made a circle with his fingers and held them together.