Musnad Abu Ya'la al-Mawsili 160:165

Abu Bakr narrated to us, Ibn Idris narrated to us, from al-Maqburi, from his grandfather, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Make the Quran clear (in its grammar/pronunciation) and seek out its wonders."