All-knowing and forgiving

One day, two friends were crossing a road in the desert together on foot. After a few hours, they had a disagreement about something and started arguing.  He slapped hard. After this incident, the friend who was slapped wrote on the desert sand:

 Today my best friend slapped me.

 Then they continued on their way until they reached a settlement. Because they were very tired, they decided to rest there for a while by the pond.

 Suddenly, the friend who was slapped slipped and fell into the pond.

 Little by little, he was drowning when his friend took his hand and saved him. After this incident, he carved this sentence on the rock by the pond:

 Today my best friend saved me from death.

 After that incident, his friend asked what did you do?

 When you were slapped, did you write that sentence on the sand, and now you carved this sentence on the stone?

 His friend replied that when we are offended by someone, we should write it on the sand so that the winds of forgiveness will carry it away.  But when someone does us good, we should engrave it on a stone so that no wind can forget it.