The sand and the stone..

Jamal and Nahim were once walking in the desert going to the east. During their journey they had an argument and Jamal slapped his friend Nahim in the face. On that day Jamal wrote in the sand: today my friend slapped me in the face. They kept on walking until they found an oasis and decided to take a bath. Jamal was stuck in the mire and almost drowning and so Nahim saved him. After Jamal recovered from drowning he wrote in a stone: today my friend saved my life. So Nahim asks Jamal after I hurt you, you wrote in the sand but when I saved you, you wrote it in the stone why? Then Jamal answered: when someone hurts you you should write it in the sand where wind and water can washed it away, but when someone do something good you should engrave it in stone where no wind can erase it.