It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "If I live until next year, in sha' Allah, I will fast the ninth (of Muharram), out of concern that the fast of the day of Ashura' might be missed by me."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10771