Imam Ali a.s teaching Dua Kumayl :
Kumayl ibn Ziyad has related: Imam caught hold of my hand and took me to the graveyard. When he had passed through the graveyard and left the city behind, he breathed a deep sigh and said:
O’ Kumayl these hearts are containers. The best of them is that which preserves (its contents). So, preserve what I say to you.