Musannaf Ibn Abi Shayba 38:180

Waki’ narrated to us, from Sufyan, from Shabib ibn Gharqada, from a man from Banu Tamim, who said: I asked Ibn Umar about the wealth of the Khawarij. He said: "There is no spoils of war in it, nor is there any misappropriation (ghulul)."