Musnad Abu Ya'la al-Mawsili 42:363

On the authority of Abu Sa’id, from the Prophet (peace and blessings of Allah be upon him), that he said: "The striver (mujahid) in the way of Allah is guaranteed by Allah: either He will suffice him with His forgiveness and His mercy, or He will return him with reward and spoils. And the likeness of the striver in the way of Allah is like the likeness of the one who fasts and stands (in prayer), who does not falter until he returns."