Abu Dawud Book 2, Hadith Number 461.

Narated By Anas ibn Malik : The Prophet (PBUH) said: The rewards of my people were presented before me, so much so that even the reward for removing a mote by a person from the mosque was presented to me. The sins of my people were also presented before me. I did not find a sin greater than that of a person forgetting the Qur’anic chapter or verse memorised by him.