It is mentioned in the book Oddatud Dai that Imam Ja’far as-Sadiq (a.s.) asked his son,
“How much amount is there for household expenses?”
“Forty Dinars,” replied the son.
Imam (a.s.) told him give in charity all the forty Dinars. The son said that there was nothing else apart from the forty Dinars. Imam (a.s.) said,
“You donate all of it in charity, the Almighty Allah will recompense it. Don’t you know that there is a key to everything? And the key to sustenance is Sadaqah.” His son Muhammad gave away the forty Dinars in charity, Ten days had hardly passed when Imam (a.s.) received four thousand Dinars. He told his son,
“O Son! We had given forty Dinars and Allah gave us four thousand in return.”