The Messenger of Allah ﷺ said, "Every deed has a period of zeal, and every period of zeal is followed by a slackening. Whoever's slackening returns him to my Sunnah has succeeded, and whoever's slackening leads him elsewhere has perished."
قال رسول الله ﷺ: إن لكل عمل شرة، ولكل شرة فترة، فمن كانت فترته إلى سنتي فقد أفلح، ومن كانت فترته إلى غير ذلك فقد هلك.
Ahmad narrated it (6764), and Ibn Khuzayma (2105) and Ibn Hibban (11) graded it sahih, all of them through the route of Husayn bin Abdul-Rahman, from Mujahid, from Abdullah bin Amr, who related it. The wording given here is Ahmad's, and its chain of transmission is sahih. It is part of a longer hadith found in the section on night prayer.