Musnad Ahmad 28:64

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Abu al-Zubayr al-Makki narrated to me, from Abu al-Abbas, the freed slave of Banu al-Dil, from Abdullah ibn Amr, who said: Some men among the companions of the Messenger of Allah, peace and blessings of Allah be upon him, were mentioned to him, who were exerting themselves intensely in worship. The Messenger of Allah, peace and blessings of Allah be upon him, said: "That is the intensity and fervor of Islam. Every intensity has a fervor, and every fervor has a period of slackening. So, whoever’s period of slackening leads him to the Book and the Sunnah, then he is upon guidance. And whoever’s period of slackening leads him to the disobedience of Allah, then that is the one who is destroyed."