Musnad Ahmad 26:208

Hajjaj narrated to us, Israel narrated to us, from Abu Ishaq, from al-Aswad, from Ibn Mas’ud, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, recited to me: {And We have certainly made the Qur’an easy for remembrance, so is there any *muddakir* (one who will remember)?} [Al-Qamar: 17]." A man said: "O Abu ‘Abd al-Rahman, is it *muddakir* or *mudhakkir*?" He said: "The Messenger of Allah, peace and blessings of Allah be upon him, recited it to me: *muddakir*."