Musnad Abu Ya'la al-Mawsili 3:140

Abu Bakr ibn Abi Shaybah narrated to us, Yahya ibn Adam narrated to us, Ubaydullah al-Ashja‘i narrated to us, from Sufyan, from Uthman ibn al-Mughirah, from Salim ibn Abi al-Ja‘d, from Ali ibn Alqamah al-Anmari, from Ali ibn Abi Talib, who said: When "O you who have believed, when you [wish to] privately consult the Messenger, present before your consultation a charity" (Al-Mujadilah: 12) was revealed, the Messenger of Allah (peace and blessings of Allah be upon him) said to me, "What do you think? 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 "Have you feared presenting before your consultation charities?" (Al-Mujadilah: 13) was revealed, up to the end of the verse. He said: Through it, Allah lightened [the burden] for this Ummah.