Musnad Ahmad 28:63

Yazid narrated to us, Muhammad ibn Ishaq informed us, from Abu al-Zubayr, from Abu al-Abbas, the freed slave of Banu al-Dil, from Abdullah ibn Amr, who said: Men who exerted themselves intensely in worship were mentioned to the Messenger of Allah, peace and blessings of Allah be upon him. He said: "That is the intensity of Islam and its fervor. For every intensity there is a fervor, and for every fervor there is a period of slackening. So, whoever’s slackening leads to moderation and the Sunnah, then that is the mother of what he is upon (i.e., he is on the right path). And whoever’s slackening leads to acts of disobedience, then that is the one who is ruined."