Musnad Abu Ya'la al-Mawsili 115:313

Ubayd Allah narrated to us, Khalid narrated to us, Sa'id narrated to us, from Qatadah, from Anas ibn Malik: That when the Messenger of Allah (peace be upon him) wanted to write to the non-Arabs, it was said to him, "They do not accept a letter unless it has a seal." He said, "So he took a seal and had engraved upon it: Muhammad is the Messenger of Allah."