Ja'far bin Abi Thawr narrates from Sayyiduna Jabir bin Samurah (may Allah be pleased with him) that he said: "The Messenger of Allah (peace and blessings be upon him) used to order us to fast on Ashura, urge us to do so and keep watch over us regarding it. Then, when Ramadan was made obligatory, he (peace and blessings be upon him) neither ordered us to do it nor forbade us from it, nor did he keep watch over us regarding it."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1856