Musannaf Ibn Abi Shayba 22:315

'Abdullah ibn Shaddad al-'Abdi narrated to us, from 'Ubaydullah ibn Sulayman al-'Abdi, from Abu Hakima al-'Abdi, who said: We were writing the Mushafs (copies of the Quran) in Kufa, and 'Ali would pass by us while we were writing. He would stand and say, "Sharpen your pen." He said: So I sharpened it, then I wrote. He then said, "Thus, illuminate what Allah has illuminated."