Al-A'mash (may Allah have mercy on him) said: I said to Shaqiq (may Allah have mercy on him): Who was the scribe of the Prophet (peace and blessings be upon him)? He said: Abdullah bin al-Arqam (may Allah be pleased with him). A letter of Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) came to us at al-Qadisiyyah, and at the bottom of it was written: "Abdullah bin al-Arqam."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20404