Caesarius of Heisterbach (ca. 1180 – ca. 1240) attributed the quote "Caedite eos. Novit enim Dominus qui sunt eius" (roughly. 'Kill them, God knows His own') to Arnaud.
That is exactly contemporary.
“That is exactly contemporary.”
No. Caesarius wrote the Dialogus Miraculorum years later. The earliest dating is that he wrote it from 1219-1223. He was not on the scene and could not have any first hand knowledge of anything said at Beziers. I am not sure if anyone actually knows when he completed the book. You might want to read William Purkis’ very recent article, ‘Crusading and Crusade Memory in Caesarius of Heisterbach’s Dialogus miraculorum’, Journal of Medieval History, Volume 39, Issue 1, 2013, pp 100-127.
Also, there’s an old article - which I do not have access to - which showed the reasons why article’s author believed the bishop could never have said it. See Tamizey de Larroque, “Rev. des quest. hist.” 1866, 1, 168-91.