Al-Hakam bin al-A'raj (may Allah have mercy on him) narrates: I went to Abdullah bin Abbas (may Allah be pleased with them both) and he was reclining on his cloak as a pillow near Zamzam. I sat with him, and he was an excellent companion. I said: Tell me about the day of Ashura. He sat up straight and then said: "About which aspect of it are you asking?" I said: About its fasting: which day should we fast? He said: "When you see the new moon of Muharram, count (the days), and when you wake on its 9th, wake up fasting." He says: I said: Is this how Muhammad (peace and blessings be upon him) used to fast? He said: "Yes."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8403