Iyad al-Ash'ari (may Allah have mercy on him) said: Abu Musa al-Ash'ari (may Allah be pleased with him) came with a delegation to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), and with him was a Christian scribe. Sayyiduna Umar (may Allah be pleased with him) was greatly impressed when he saw his memory. He said: Tell your scribe to read us a letter. Abu Musa (may Allah be pleased with him) said: He is a Christian; he does not enter the mosque. Sayyiduna Umar (may Allah be pleased with him) rebuked him and was about to punish him, and said: Do not honour them when Allah has humiliated them, do not bring them near when Allah has kept them distant, and do not trust them when Allah has declared them untrustworthy.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20409