Musnad Ahmad

Hakam bin A'raj says that once I presented myself before Sayyiduna Ibn Abbas (may Allah be pleased with them both), he was sitting near the well of Zamzam, reclining on his cloak. I also went and sat with him; he was the best of companions. I said to him, "Tell me something about the Day of Ashura." He said, "In what regard are you asking about it?" I said, "Regarding fasting, that is, on which day should I fast?" He said, "When you see the crescent of Muharram, keep counting its days. When the morning of the ninth day comes, then fast." I asked, "Did the Prophet (peace and blessings be upon him) fast in this manner?" He said, "Yes."