Ziyad bin Hudayr al-Asadi said: Sayyiduna Ali (may Allah be pleased with him) said: If I remain alive for the Christians of Banu Taghlib, I will certainly kill their fighting men and take their offspring as captives, for I wrote the agreement between the Prophet (peace and blessings be upon him) and them that "they will not make their children Christians."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18800