Musnad Ahmad 907:102

Hashim ibn al-Qasim narrated to us, Shayban narrated to us—I think from 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 fast Ashura], nor did he forbid us from it, nor did he check on us regarding it."