Musnad Abu Ya'la al-Mawsili 160:162

Muhammad ibn Bakkar narrated to us, Abu Ma'shar narrated to us, from Sa'id, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, for this Qur'an there is an intensity, and for the people regarding it there is a period of slackening. So whoever’s slackening is toward moderation, then how excellent that is. And whoever’s slackening is toward turning away, then those are the ones who are ruined."