Musannaf Ibn Abi Shayba 22:11

Abu Dawud al-Tayalisi narrated to us, from Shu'bah, from Abu Raja', who said: I asked Muhammad about dotting the Mushafs (copies of the Quran). He said: "I fear that you might add to the letters or subtract from them." And I asked al-Hasan, and he said: "Has it not reached you what Umar wrote? That you should learn Arabic, improve your expression, and gain understanding in the religion."