He said, "The Messenger of Allah ﷺ commanded me to learn the writing of the Jews for him. He said, 'By Allah, I do not trust the Jews with my correspondence.' So I learned it, and not even half a month passed before I had mastered it. I would write for him whenever he wrote, and read for him whenever something was written to him."
قال أمرنى رسول الله ﷺ فتعلمت له كتاب يهود وقال: إني والله ما آمن يهود على كتابي فتعلمته فلم يمر بي إلا نصف شهر حتى حذقته، فكنت أكتب له إذا كتب، وأقرأ له إذا كتب إليه.
Narrated by Abu Dawud (3645) - and this is his wording, Tirmidhi (2715), and Ahmad (21618) via Abdul-Rahman bin Abu Zinad, from his father, from Kharija bin Zayd bin Thabit, from Zayd bin Thabit, who said, and then he related it.Bukhari mentioned it as a suspended chain in Ahkam (7195), from Kharija bin Zayd, in abridged form. Its chain of transmission is hasan because of Abdul-Rahman bin Abu Zinad, whose hadith is hasan.Tirmidhi said, this is a hasan sahih hadith.