Musannaf Ibn Abi Shayba 11:28

Yazid ibn Harun narrated to us, [saying] Muhammad ibn Ishaq informed us, from Muhammad ibn Ibrahim, from Muhammad ibn Abdullah ibn Atik, from his father, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Whoever goes out striving in the way of Allah—then he gathered his three fingers and said: 'And where are the strivers?'—then he fell from his mount and died, his reward has fallen upon Allah. Or a beast stung him, his reward has fallen upon Allah. And whoever dies a natural death, his reward has fallen upon Allah. And whoever is killed by a sudden, fatal blow, he has merited the return [to Paradise]."