Musnad Ahmad 788:235

Yunus ibn Muhammad narrated to us, he said: Abu Layla Abdullah ibn Maysarah narrated to us, from Mazidah ibn Jabir, he said: My mother said: I was in the Mosque of Kufa during the caliphate of Uthman, may Allah be pleased with him, and Abu Musa al-Ash'ari was over us. She said: I heard him saying: "Indeed, the Messenger of Allah, may Allah bless him and grant him peace, commanded the fasting of Ashura, so fast it."