Hadith 2780
It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "It is very bad for any one of you to say, 'I have forgotten such-and-such verse.' Rather, (he should say) 'It was made to be forgotten,' and continue reciting the Noble Qur'an, for its departure from the hearts of people is swifter than the escape of a camel."