Musnad Ahmad 1118:31

Ali ibn Ishaq narrated to us, he said: Abdullah—meaning Ibn al-Mubarak—informed us, he said: Haywah ibn Shurayh informed us, he said: Abu Hani al-Khawlani informed me that he heard Amr ibn Malik al-Janbi saying: I heard Fudalah ibn Ubayd saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The mujahid (struggler) is the one who struggles against his own self for the sake of Allah."