Musnad Ahmad

It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "Yajuj and Majuj dig through the wall of Dhul-Qarnayn every day, and when they have dug so much that they can see the rays of the sun, their leader says, 'Go back now, you will break it down tomorrow.' But when they return the next day, it is stronger than before. This will continue until their appointed time comes. When Allah wills to unleash them upon the people, they will dig through it, and when they have dug so much that they can see the rays of the sun, their leader will say, 'Go back now, tomorrow, insha’Allah, you will break it down.' So when they return the next day, the wall will be as they left it, and they will break it down and rush upon the people. They will drink up the water from the springs and finish it. People, out of fear of them, will lock themselves in their forts. Then they will shoot their arrows towards the sky, and the arrows will return to them blood-stained. They will say, 'We have prevailed over the people of the earth and have dominated those of the sky as well.' After that, Allah will send a worm in their necks, near the nape, which will cause all of them to die. The Noble Prophet (peace and blessings be upon him) said: 'By Him in Whose hand is the soul of Muhammad (peace and blessings be upon him), the insects and animals of the earth will become so satiated and healthy by feeding on their flesh and blood.'"