Musannaf Ibn Abi Shayba 22:87

Waki’ narrated to us, from Ibrahim bin Yazid, from al-Walid bin ‘Abd Allah bin Abi Mughith, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The sins were presented to me, and I did not see anything among them greater than [the sin of] a bearer of the Qur’an who abandons it."