Sayyiduna Anas bin Malik (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "The rewards of my Ummah were presented to me, even the reward of someone removing a speck of straw from the mosque. The sins of my Ummah were also presented to me, and the greatest sin was memorising a surah or a verse of the Qur'an and then forgetting it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4312