Musannaf Ibn Abi Shayba 27:488

Yahya ibn Adam narrated to us, he said: Ubaydullah al-Ashja‘i narrated to us, from Sufyan, from Sa‘d, from Uthman ibn al-Mughirah al-Thaqafi, from Salim ibn Abi al-Ja‘d, from Ali ibn Alqamah al-Anmari, from Ali, who said: When this verse was revealed: "O you who have believed, when you [wish to] privately consult the Messenger, present before your consultation a charity," the Messenger of Allah (peace and blessings of Allah be upon him) said to me: "What do you see? A dinar?" I said: "They cannot afford it." He said: "Then how much?" I said: "A barleycorn." He said: "You are indeed stingy." He said: Then [the verse] was revealed: "Have you feared presenting before your consultation charities?" [58:13] until the end of the verse. He said: So Allah lightened [the burden] for this nation.