Musannaf Ibn Abi Shayba 38:41

Qabisah narrated to us, he said: Sufyan narrated to us, from Abu Ishaq, from Khumayr (or Humayr) ibn Malik, who said: Ammar said to Ali on the day of the Camel: "What is your opinion regarding the enslavement of the offspring?" He said: He replied, "We only fought those who fought us." He said: "Had you said other than this, we would have opposed you."