Musnad Ahmad 30:3277

Hajjaj narrated to us, he said: Layth narrated to me, he said: Sa'id narrated to me, from 'Ata' ibn Mina', the freed slave of Ibn Abi Dhubab, that he heard Abu Hurayrah saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Allah, the Almighty and Majestic, has guaranteed for the one who goes out in His cause—nothing drives him out except belief in Me and striving in My cause—that He is a guarantor for him that He will cause him to enter Paradise by whichever of the two states he is in: either by being killed, or by natural death; or I will return him to his dwelling from which he went out, having attained what he attained of reward or spoils."