Musnad Abu Ya'la al-Mawsili 3:72

Ubayd Allah narrated to us, Abd al-Rahman ibn Uthman al-Bakrawi narrated to us, al-Kalbi narrated to us, from al-Asbagh ibn Nubatah, from Ali, who said: I witnessed the Prophet, may Allah bless him and grant him peace, making a peace treaty with the Christians of Banu Taghlib on the condition that they would not baptize their children; and if they did so, then the covenant is disavowed from them. He said: Ali then said, "By Allah, they have indeed done so. By Allah, if the matter is completed for me, I will surely kill their fighting men and take their offspring as captives."