There isn't a verse in the Bible that explicitly says one third of the angels fell. But generally, the verse used to support this view is found in the book of Revelation dealing with the great red dragon that many Christian commentators believe references the devil.
Rev. 12:4, “And his tail swept away a third of the stars of heaven, and threw them to the earth. And the dragon stood before the woman who was about to give birth, so that when she gave birth he might devour her child.”