Musnad Ahmad 907:202

Hashim ibn al-Qasim narrated to us, Shayban narrated to us, from al-Ash'ath, from Ja'far ibn Abi Thawr, from Jabir ibn Samurah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to command us to fast the day of Ashura, and he would urge us toward it, and he would check on us regarding it. But when Ramadan was made obligatory, he did not command us to do it, nor did he forbid us from it, and he did not check on us regarding it."