Musnad Ahmad 30:2121

Musa ibn Dawud narrated to us, he said: Ibn Lahi'ah narrated to us, from Musa ibn Wardan, from Abu Hurairah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever dies while stationed (in Ribat) for the sake of Allah, he is protected from the trial of the grave, and he is secured from the Greatest Terror. And his provision is brought to him, morning and evening, from Paradise, and the reward of the one stationed (in Ribat) is written for him until the Day of Resurrection."