While there is no verse that says "a third of the angels fell from heaven," some verses, when put together, lead us to that conclusion. Sometime after their creation, and most certainly after the sixth day when everything was declared “very good” (Genesis 1:31), Satan rebelled and was cast out of heaven. “How you have fallen from heaven, O morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations!” (Isaiah 14:12). Jesus said, “I saw Satan fall like lightning from heaven” (Luke 10:18), and in the book of the Revelation Satan is seen as “a star that had fallen from the sky to the earth” (Revelation 9:1).
We are also told that one third of an “innumerable company of angels” (Hebrews 12:22) chose to rebel with him. John saw this great wonder in heaven, “…an enormous red dragon…His tail swept a third of the stars out of the sky and flung them to the earth…the great dragon was hurled down—that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him” (Revelation 12:3–9).
Since Satan is referred to as a star which fell or was cast down to earth, and Revelation 12:4 says a third of the stars were cast out with him, then the conclusion is that the stars in Revelation 12 refer to fallen angels, fully one third of the heavenly host. If the one-third number is in fact accurate, what assurance that is! Two thirds of the angels are still on God’s side, and for followers of Christ, they are on our side as well.