Sahih Muslim 46

Qutaybah bin Sa’īd narrated to us, Jarīr narrated to us, on authority of Mughīrah, on authority of Ibrāhīm [bin Yazīd an-Nakhā’ī], he said, Alqamah said: ‘I memorized the Qur’ān in two years’. Al-Hārith said: ‘The Qur’ān is easy; the revelation [i.e. the secret revelation to Alī in the belief of the Shī’ah] is more difficult ’.