Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: None of you should say, "I have taken the whole Qur'an," for how does he know what the whole Qur'an is! Much of the Qur'an has passed away from it. Rather, he should say: We have taken from it that which has appeared.
Hadith Referenceسنن سعید بن منصور / كتاب التفسير / 140