Musnad Ahmad 30:2870

Ishaq narrated to us, Malik narrated to me, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "The likeness of the mujahid in the way of Allah is like the likeness of the one who is constantly fasting and standing in prayer, who does not falter from fasting or prayer until he returns."