Ubadah bin al-Nu'man al-Taghlibi said that he said to Sayyiduna Umar (may Allah be pleased with him): O Commander of the Believers, you know the might of Banu Taghlib; they are positioned facing the enemy, and if they back the enemy against you, dealing with them will become hard. If you see fit, give them something. Sayyiduna Umar (may Allah be pleased with him) said: I will give something. The narrator said: Sayyiduna Umar (may Allah be pleased with him) made peace with them on the condition that they would not dye their children in Christianity and that the sadaqah on them would be doubled. Ubadah used to say: They have done it, and they have no covenant.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18796