Musannaf Ibn Abi Shayba 3:6178

Waki’ narrated to us, he said: ‘Abd al-Malik ibn Shaddad al-Awdi narrated to us, from ‘Ubayd ibn Sulayman al-‘Abdi, from Abu Hakimah 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 say, “Sharpen your pen.” He said: So I cut [the tip] of it, then I wrote. Then he said, “Thus, illuminate what Allah the Exalted has illuminated.”