Al-Sha'bi (may Allah have mercy on him) narrates from Qadi Shurayh (may Allah have mercy on him) that a case came to him about a child, one of whose parents was a Christian. He said: The Muslim parent has the greater right to the child.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21293