Where in the Bible does it state that the devil was a fallen angel?

Answer 1

Satan and the "one-third" of God's created angels who followed him in his rebellion against God's Government were "cast back to earth" from whence they launched their attack.

"...the great Dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: HE WAS CAST OUT [of heaven] INTO THE EARTH, and HIS ANGELS WERE CAST OUT WITH HIM." (Rev.12:9)

Jesus Christ [the Almighty Creator God of the Old Testament - John 1:3 & 10] says: "...I beheld Satan as lightning FALL FROM HEAVEN." (Luke 10:18)

Jesus was there. He saw it... and is an eye-witness to it.

Answer 2

"How you are fallen from heaven, O Day Star [Lucifer], son of Dawn! How you are cut down to the ground, you who laid the nations low!" (Isaiah 14:12, ESV)

And he said to them, "I saw Satan fall like lightning from heaven." (Luke 10:18, ESV)

Now war arose in heaven, Michael and his angels fighting against the dragon. And the dragon and his angels fought back, but he was defeated, and there was no longer any place for them in heaven. And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world-he was thrown down to the earth, and his angels were thrown down with him. (Revelation 12:7-9, ESV)