Musannaf Ibn Abi Shayba 28:1142

Hussain bin Ali narrated to us, from Za'idah, from Abd al-Malik bin Abjar, who said: A man called Sabih said: We were a group of the Fatah with Ali. He said: And Ali was an experienced man. He said: And he used to say, "War is deception." He said: So he would reach the rock and say, "Allahu Akbar, Allah and His Messenger have spoken the truth," [referring to] the rock. He said: So we would think that it was something that had been said to him. He said: Then he would reach the Tigris and say, "Allahu Akbar, Allah and His Messenger have spoken the truth," [referring to] the Tigris. He said: So we would think that it was something that had been said to him.