Imam Sadiq (a.s) had many sons, one of whom was named Muhammad. Occasionally it would so happen that his income would exceed his expenses. Once, Imam Sadiq (a.s) inquired: “How much have you managed to save?” He replied: “Forty dinars.” The Imam (a.s) said: “Give it as charity in the way of Allah. He said: This is all that I have with me and if I were to give this in charity there would be nothing left for me.”
The Imam (a.s) insisted: “Go and give it in charity and Allah shall compensate it for you. Do you not know that for everything there is a key and the key to sustenance is charity?”
Muhammad, heeding his father’s advice, gave the forty dinars in charity. Hardly ten days had passed by since this episode when a sum of four thousand dinars was presented to the Imam (a.s). Turning to Muhammad, he (a.s) said: “My Son! We gave forty dinars in the path of Allah and He gave us four thousand dinars (a hundred times) in return.”( Hikayat-ha-e-Shanidani, vol. 4, pg. 92; Al-Kafi, vol. 4, pg. 9 )