Abu al-Aswad said: A case was brought to Sayyiduna Mu'adh (may Allah be pleased with him) concerning a man who had died on a religion other than Islam, and whose son was a Muslim. Sayyiduna Mu'adh (may Allah be pleased with him) made him (the son) his heir and said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Islam increases and does not decrease."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12464