Musnad Ahmad 1153:2050

Rawh narrated to us, he said: Muhammad ibn Abi Hafsah narrated to us, he said: Ibn Shihab narrated to us, from Urwah, from Aisha, she said: They used to fast the day of Ashura before Ramadan was made obligatory, and it was a day in which the Kaaba was covered. When Allah, the Almighty and Majestic, made Ramadan obligatory, the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever wishes to fast it, let him fast it, and whoever wishes to leave it, let him leave it."