Sayyiduna Tamim al-Dari (may Allah be pleased with him) said: I said: "O Messenger of Allah, a disbeliever accepts Islam at the hands of a Muslim; what is the Sunnah regarding him?" He (peace and blessings be upon him) said: "He (the Muslim) is the person with the greatest right over him, in his life and in his death."
Hadith Referenceالمعجم الكبير للطبراني / باب التاء / 1260