Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, Gog and Magog dig (the great barrier) every day. When they see the rays of the (setting) sun, their leader says to them: 'Go back now, you will finish digging it tomorrow.' But Allah returns it to a harder state than before. (This will continue) until their appointed time comes and Allah wills to send them upon the people. They will begin to dig until the rays of the (setting) sun appear to them. Their leader will say: 'Go back now, if Allah wills, we will complete its digging tomorrow.' (Before this, they will not have said 'if Allah wills', only this time they will say it.) When they come (in the morning), they will find it in the same state as they left it. They will dig it completely and emerge upon the people. They will dry up the water, and people will take refuge in strong fortresses. They will shoot their arrows towards the sky, and after doing so, they will say: 'We have subdued the people of the earth and have also overpowered the people of the sky.' (Eventually) Allah will create a worm in their necks, by which they will die." The Messenger of Allah (peace and blessings be upon him) said: "By the One in Whose hand is my soul! The animals of the earth will become fat and healthy by eating their flesh."