Sahih al-Bukhari 3831

Musaddad narrated to us: Yahya narrated to us. Hisham said: My father narrated to me, from ʿAʾisha, may Allah be pleased with her, who said: “ʿAshuraʾ was a day that Quraysh used to fast in pre-Islamic times, and the Prophet used to fast it. When he came to Medina, he fasted it and commanded that it be fasted. When Ramadan was prescribed, whoever wished fasted it, and whoever wished did not fast it.”