Ali said, "If I live long enough to deal with the Christians of Banu Taghlib, I will surely kill their fighting men and take their offspring as captives, for I wrote the agreement between them and the Prophet ﷺ stipulating that they would not raise their children as Christians."
حدثنا العباس بن عبد العظيم حدثنا عبد الرحمن بن هانئ أبو نعيم النخعي أخبرنا شريك عن إبراهيم بن مهاجر
قال علي لئن بقيت لنصارى بني تغلب لأقتلن المقاتلة ولأسبين الذرية فإني كتبت الكتاب بينهم وبين النبي ﷺ على أن لا ينصروا أبناءهم
No references are available.
[Its chain of transmission is daif]. Abu Naeem Nakhai is daif, the majority of scholars declared him daif, and Sharik the judge is a mudallis who narrated using "an" (indicating possible concealment in the chain).