Dawud bin Kurdus narrates from Sayyiduna Umar (may Allah be pleased with him) that he made peace with Banu Taghlib on the condition that they would not dye any child in their religion, that the sadaqah on them would be doubled, and that they would not be compelled to any religion other than their own. Dawud said: Banu Taghlib had no covenant, because they had dyed their children in Christianity.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18795