AN4.147 — Dutiyakālasutta
Disciples, there are these four times which, when developed and pursued correctly, gradually lead to the destruction of the taints. What are the four? Listening to the Dhamma at the right time, discussing the Dhamma at the right time, thorough investigation at the right time, and insight at the right time. These are the four times which, when developed and pursued correctly, gradually lead to the destruction of the taints.
Just as when rain falls on the mountain top, the water flows down and fills the mountain clefts, gullies, and channels; when these are full, they fill the small ponds; when the small ponds are full, they fill the large lakes; when the large lakes are full, they fill the small rivers; when the small rivers are full, they fill the great rivers; and when the great rivers are full, they fill the ocean. In the same way these four times, when developed and pursued correctly, gradually lead to the destruction of the taints.