It is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Every child is born upon the fitrah (natural disposition) until his tongue begins to speak; then his parents make him a Jew and a Christian."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 824