Abu Dawud Book 8, Hadith Number 2518.

Narated By Ubaydullah ibn Khalid as-Sulami : The Apostle of Allah (PBUH) made a brotherhood between two men, one of whom was killed (in Allah’s path), and a week or thereabouts later the other died, and we prayed at his funeral). The Apostle of Allah (PBUH) asked: What did you say? We replied: We prayed for him and said: O Allah, forgive him, and join him to his companion. The Apostle of Allah (PBUH) said: What about his prayers since the time the other died, and his fasting since the time the other died–the narrator Shu’bah doubted the words, “his fasting–and his deeds since the time the other died. The distance between them is just like the distance between heaven and earth.