Musnad Ahmad 323:1

Yazid ibn Harun narrated to us, he said: Muhammad ibn Ishaq informed us, from Muhammad ibn Ibrahim ibn al-Harith, from Muhammad ibn Abdullah ibn Atik, one of the Banu Salamah, from his father, Abdullah ibn Atik, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Whoever leaves his house as a mujahid in the path of Allah, the Almighty and Majestic," then he gestured with these three fingers of his—the middle finger, the index finger, and the thumb—and he gathered them together and said: "And where are the mujahideen?" Then he fell from his mount and died, his reward has fallen upon Allah, the Almighty and Majestic. Or a creature stung him and he died, his reward has fallen upon Allah. Or he died a natural death, his reward has fallen upon Allah, the Almighty and Majestic. By Allah, it is a phrase I did not hear from anyone among the Arabs before the Messenger of Allah, peace and blessings of Allah be upon him: "He died, so his reward has fallen upon Allah." And whoever is killed by a sudden death (qa'san) has earned the return.