Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that it was mentioned to the Prophet (peace and blessings be upon him) that the Jews fast on the day of Ashura. So the Prophet (peace and blessings be upon him) said: "If we live, we will differ from them and fast on the ninth (as well)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11219