Musnad Ahmad 30:3501

Rawh narrated to us, Sa'id ibn Abi 'Arubah narrated to us, from Qatadah, Abu Rafi' narrated to us, from Abu Hurayrah, from the Messenger of Allah (peace and blessings of Allah be upon him), who said: "Indeed, Gog and Magog dig at the barrier every day, until when they are about to see the rays of the sun, the one who is over them says: 'Go back, for you will dig it tomorrow.' So they return to it as it was, or even stronger. Until, when their time is reached and Allah intends to send them upon the people, they will dig until, when they are about to see the rays of the sun, the one who is over them says: 'Go back, for you will dig it tomorrow, if Allah wills,' and he makes an exception. So they return to it, and it is in the same state as when they left it. They dig it and emerge upon the people, and they dry up the waters. The people will fortify themselves against them in their strongholds, and they will shoot their arrows toward the sky. They will return with something like the appearance of blood upon them, and they will say: 'We have conquered the people of the earth and overcome the people of the heaven.' Then Allah will send upon them *naghaf* (larvae) in their napes, and He will kill them by it." The Messenger of Allah (peace and blessings of Allah be upon him) then said: "By the One in Whose hand is the soul of Muhammad, the beasts of the earth will grow fat and become thankful, out of gratitude, from their flesh and their blood."