Ibn Hanbal writes in his Musnad that when the verse:
O you who believe! When you consult the Apostle, then offer something in charity before your consultation; (Surah Mujadilah 58:12)
was revealed, the Prophet told Ali (a.s), “Go and command those people to pay alms (Sadaqah).” “How much should I ask them to pay?” asked Ali (a.s). “One Dinar”, replied the Prophet (S). Ali (a.s) said, “They are unable to pay this much.” “Half a Dinar”, said the Prophet. “They cannot even pay this much” said Ali (a.s). The Prophet said, “A grain of gold.” Ali (a.s) said, “Maybe they cannot even afford this.” The Prophet said, “O Ali! You are very kind to the people. All right! Tell them to give only one dirham.” Amir’ul-Mu’minin (a.s) used to say, “The reduction in this command was due to me.”
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams