Musannaf Ibn Abi Shayba 33:326

'Abd al-Rahman ibn Muhammad al-Muharibi narrated to us, from Harun ibn 'Antara, from 'Abd al-Rahman ibn al-Aswad, from his father, who said: 'Abdullah ibn Mas'ud said: "These hearts are merely vessels, so occupy them with the Qur'an and do not occupy them with anything else."