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: "By the One in whose hand is my soul, no soul is born except upon the fitrah (natural disposition), until its tongue begins to speak."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 829