Zadhan narrates from Sayyiduna Salman (may Allah be pleased with him) that no one was greater in sanctity than the Prophet (peace and blessings be upon him). Whenever the Companions of the Messenger of Allah (peace and blessings be upon him) (may Allah be pleased with them) wrote to him, they would write: "From so-and-so to Muhammad, the Messenger of Allah (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20424