Musnad Ahmad 938:43

Sulayman ibn Dawud narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Kharijah ibn Zayd, that his father Zayd informed him: When the Prophet, peace and blessings of Allah be upon him, arrived in Medina, Zayd said: I was taken to the Prophet, peace and blessings of Allah be upon him, and he was impressed by me. They said: "O Messenger of Allah, this is a boy from Banu al-Najjar, who has with him more than ten surahs of what Allah has revealed to you." The Prophet, peace and blessings of Allah be upon him, was pleased by that and said: "O Zayd, learn the writing of the Jews for me, for by Allah, I do not trust the Jews with my writing." Zayd said: So I learned their writing for him. Not even fifteen nights passed by me until I had mastered it, and I would read their letters to him when they wrote to him, and I would answer on his behalf when he wrote.