Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: A man's letter came to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) instructed Abdullah bin al-Arqam (may Allah be pleased with him): "Reply on my behalf." He wrote the reply and read it to him (peace and blessings be upon him), and he (peace and blessings be upon him) said: "You have written correctly and done well. O Allah, grant him success." When Sayyiduna Umar (may Allah be pleased with him) became caliph, he used to consult him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20403