Daniel 7:19
Then it was my desire to have certain knowledge about the fourth beast, which was different from all the others, a cause of great fear, whose teeth were of iron and his nails of brass; who took his food, crushing some of it to bits and stamping on the rest with his feet;
Daniel 7:7
After this, in my vision of the night, I saw a fourth beast, a thing causing fear and very troubling, full of power and very strong; and it had great iron teeth: it took its food, crushing some of it to bits and stamping down the rest with its feet: it was different from all the beasts before it; and it had ten horns.
Daniel 2:40-43
And the fourth kingdom will be strong as iron: because, as all things are broken and overcome by iron, so it will have the power of crushing and smashing down all the earth.