Sahih Muslim Book 4, Hadith Number 1745.

Chapter : Excellence of the Hifz; (one who commits the Qur’an to memory) of the Qur’an.

‘Aisha reported Allah’s Messenger (may peace be upon him) (as saying): One who is proficient in the Qur’an is associated with the noble, upright, recording angels; and he who falters in it, and finds it difficult for him, will have a double reward.