Sayyiduna Salman (may Allah be pleased with him) said: "No one was held in greater sanctity than the Messenger of Allah (peace and blessings be upon him), and when his (peace and blessings be upon him) Companions wrote to him, they would write: From so-and-so to Muhammad, the Messenger of Allah (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6083