Wait, I misread that. I thought you were talking about the Rubicon crossing (that's mentioned in Suetonius with the famous phrase iacta alea est). I'm not sure about the Rhine, sorry.
OK so I looked it up and if this is what you're talking about, then it's mentioned in Book 4 (chapter 17 et seqq.) and Book 6 (chapter 9) of De Bello Gallico.
