Qatadah (may Allah have mercy on him) said that Abu Ubaydah bin al-Jarrah and Khalid bin al-Walid (may Allah be pleased with them both) both wrote to Umar bin al-Khattab (may Allah be pleased with him) and began with their own names.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20423