Musannaf Ibn Abi Shayba 22:247

Waki’ narrated to us, from Sufyan, from Mansur, who said: Ibn Abi Layla narrated to us, from al-Hakam, from Miqsam, from Ibn Abbas regarding the saying of Allah, the Exalted: "And recite the Qur'an with measured recitation," he said: "Clarify it with clarification."