Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) says that Sayyiduna Umar (may Allah be pleased with him) ordered him to send him an account of what he had collected and what he had given out, written on a single hide. Sayyiduna Abu Musa (may Allah be pleased with him) had a Christian scribe, who brought it to him. Sayyiduna Umar (may Allah be pleased with him) was amazed and said: "This man is indeed a good keeper of records." And he said: "We have a letter in the mosque that has come from Syria; call him so that he may read it." Sayyiduna Abu Musa (may Allah be pleased with him) said: "He cannot enter the mosque." Sayyiduna Umar (may Allah be pleased with him) said: "Is he in a state of major ritual impurity?" Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) said: "No, rather he is a Christian." Sayyiduna Abu Musa (may Allah be pleased with him) says: Sayyiduna Umar (may Allah be pleased with him) rebuked me, struck my thigh and said: "Remove him," and he recited this verse: "O you who believe! Do not take the Jews and the Christians as allies. They are allies of one another. And whoever among you takes them as allies is indeed one of them. Indeed, Allah Almighty does not guide the wrongdoing people." [Surah al-Ma'idah: 51]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18727