This video has never been released publicly. It's the personal video of someone.

After the first plane hit one of the towers, this cameraman set up his home video camera on a balcony 1 block from the WTC and left the room to let it record, unknowing a 2nd plane would hit and he would catch it on tape.

For information and to download original higher res version:

http://jeremymorrison.com/wtcplane.html