‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "If I were to live till next year, I would definitely fast the ninth day."
As related in the Hadīth, the Prophet (may Allah's peace and blessings be upon him) resolved to fast the ninth day of Muharram along with the tenth day (‘Ashūrā') if he would live till the following year. Thereby, he aimed to contradict the Jews. However, he died before next Muharram. So it is Sunnah to fast the ninth day, even though the Prophet (may Allah's peace and blessings be upon him) could not fast it. Indeed, whatever the Prophet (may Allah's peace and blessings be upon him) determined to do is regarded as Sunnah. By fasting the ninth day along with the tenth, the Prophet (may Allah's peace and blessings be upon him) aimed at contradicting the Jews, who fast the tenth day only. It is also said that fasting the ninth day is a way of caution ensuring that ‘Ashūrā' is not missed. The first opinion is stronger, however, because it is based on an explicit text. And Allah knows best.