Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) instructed me to learn the writing of the Jews, and he said: "By Allah, I do not feel safe with the Jews in the matter of my writing." I learned the writing within half a month.
Abu Dawud (may Allah have mercy on him) said: I became proficient in it within half a month. My father said: I used to write for him (peace and blessings be upon him) when a letter was to be written, and I would read for him when a letter came.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20407