Abu Dawud Book 19, Hadith Number 3638.

Narated By Zayd ibn Thabit : The Apostle of Allah (PBUH) ordered me (to learn the writing of the Jews), so I learnt for him the writing of the Jews. He said: I swear by Allah, I do not trust Jews in respect of writing for me. So I learnt it, and only a fortnight passed that. I mastered it. I would write for him when he wrote (to them), and read to him when something was written to him.