Musnad Abu Ya'la al-Mawsili 115:106

Nasr ibn Ali narrated to us, Nuḥ ibn Qays narrated to us, from his brother Khālid ibn Qays, from Qatādah, from Anas: That the Prophet, may Allah bless him and grant him peace, wrote to Bakr ibn Wā’il: "From Muḥammad, the Messenger of Allah, to Bakr ibn Wā’il: Submit [to Islam] and you will be safe." We did not find anyone who could read it except a man from Banū Ḍubay’ah, and they are called the "Banu al-Kātib" (Sons of the Scribe).