Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: Keep these Mushafs firmly in memory; by Allah, the Qur'an escapes from people's breasts more quickly than animals escape from their tethers. (So keep reciting it.) Let none of you ever say: "Nasitu ayata kayta wa kayt" (I have forgotten such-and-such a verse), for the Messenger of Allah (peace and blessings be upon him) said: "It is made to be forgotten."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4052