Musannaf Ibn Abi Shaybah

Hazrat Abu Wa'il narrates that Hazrat Abdullah bin Mas'ud said: Take care of this Qur'an, for it slips away more quickly from the hearts than a camel does from its ropes. And he said that the Messenger of Allah, peace and blessings be upon him, said: Woe to the one who says, "I have forgotten such-and-such verse." He did not forget it, rather it was made to be forgotten by him.