“I once asked abu ‘Abd Allah (a.s.), about a man who dies and has a son who does not know if his father has performed al-Hajj or not. He (the Imam) said, ‘He performs al-Hajj on his behalf, if his father has performed al-Hajj, it is written as optional al-Hajj for him and an obligatory al-Hajj for his son; and if his father has not performed al-Hajj, it is written obligatory for his father and optional for the son.’”