The Prophet ﷺ wrote to Bakr bin Waail, "From Muhammad, the Messenger of Allah ﷺ, to Bakr bin Waail. Accept Islam and you will be safe." We could not find anyone able to read it except a man from Banu Dubayah, and so they came to be called Banu al-Katib.
أن النبي ﷺ كتب إلى بكر بن وائل: من محمد رسول الله ﷺ إلى بكر بن وائل، أسلموا تسلموا فما وجدنا من يقرؤه إلا رجل من بني ضبيعة، فهم يسمون بني الكاتب.
Bazzar narrated it in Kashf Astar (1670), and Abu Yaala (2947), and Ibn Hibban graded it sahih (6558), all of them through the route of Nasr bin Ali, who said, Nuh bin Qays narrated to us, from his brother Khalid bin Qays, from Qatada, from Anas, and he mentioned it. Its chain is hasan because of Nuh bin Qays and his brother Khalid, for the two of them make the hadith hasan.Similar in meaning is what was narrated from Marthad bin Zibyan, who said, "A letter came to us from the Messenger of Allah, and we could not find anyone to read it to us until a man from Banu Dubayah read it, saying, 'From the Messenger of Allah to Bakr bin Waail, embrace Islam and you will be safe.'"Ahmad narrated it (20667) from Yunus and Husayn, who both said, Shayban narrated to us, from Qatada, from Mudarib Hazn Ajli, who said, and Marthad bin Zibyan narrated, and he mentioned it.Haythami said in Majma (5/305), "Ahmad narrated it, and its narrators are the narrators of the Sahih." I say, it contains Mudarib bin Hazn, whom none of the authors of the six books narrated from except Ibn Majah, and I did not find anyone who declared him trustworthy other than Ibn Hibban and Ajli, and for that reason Ibn Hajar said, "Acceptable," meaning when corroborated, and I did not find any corroboration for him.Ibn Sakan mentioned this hadith in mualaq form, and said, "It is mursal." And he said about Marthad, "He is not known among the Companions." This is as found in Isabah (10/104).