إِنَّ لِكُلِّ شَيْءٍ شِرَّةً، وَلِكُلِّ شِرَّةٍ فَتْرَةً
Translation: Every activity has a period of vigour, and every period of vigour is followed by a period of slackening.
Source: الترمذي
People sometimes begin worship, study, or service with great energy, only to slow down later. شِرَّةً means a burst of enthusiasm; فَتْرَةً is a later reduction in intensity. These changes are familiar, but excess at the start and abandonment of obligations later are both problems.
In لِكُلِّ شَيْءٍ, the preposition lām means “for,” and كُلِّ is a noun in an iḍāfah construction. In إِنَّ لِكُلِّ شَيْءٍ شِرَّةً, شِرَّةً is the delayed noun of inna, while the preceding prepositional phrase functions as the predicate. The second clause follows the same pattern.
A sustainable routine is more valuable than an intense beginning that causes one to neglect essential duties.