We were commanded to fast on Ashura before Ramadan was prescribed. When Ramadan was prescribed, we were no longer commanded to fast it.
أمرنا بصيام عاشوراء قبل أن ينزل رمضان، فلما نزل رمضان لم نؤمر به.
Tabari narrated it in Tahzib Athar (634) from Ibn Bashar, who said, Muslim bin Ibrahim narrated to me, who said, Hammam bin Yahya narrated to us, who said, Qatada narrated to us, from Abu Hassan, that Ammar bin Yasir said, and then he mentioned it. Its isnad is sahih.Tabarani also narrated it in al-Kabir, as Haythami mentioned in Majma al-Zawaid (3/188), and he said, "Its narrators are the narrators of the sahih." However, I have not been able to locate its isnad, because it falls within the missing portion, and it appears most likely that its isnad meets up with the isnad of Tabari.