Abu al-Aswad al-Du'ali (may Allah have mercy on him) said: A man from among us, who was a Jew - or a Christian - died and left behind a Muslim son. The matter was taken to Sayyiduna Mu'adh (may Allah be pleased with him), and he made him an heir. He said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Islam increases and does not decrease."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17475