It is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Every child is born upon the fitrah (natural disposition) until he expresses himself. His parents make him a Jew or a Christian."
The old position of Imam al-Shafi'i (may Allah have mercy on him) is that the saying of the Prophet (peace and blessings be upon him), "Every child is born upon the fitrah upon which Allah Almighty created the creation," means that the Messenger of Allah (peace and blessings be upon him) held that, as long as they have not expressed themselves in speech and chosen one of the two positions, faith or disbelief, they have no ruling of their own; their ruling follows that of their fathers. So whatever their fathers were on the day they were born, that is their state: if a believer, then he is upon his faith, and if a disbeliever, then upon his disbelief.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12143