I came to Ibn Abbas while he was reclining near Zamzam, so I sat with him. He was an excellent companion. I said, "Tell me about the day of Ashura."He asked, "What about it do you wish to know?"I said, "About fasting on it."He said, "When you see the crescent of Muharram, start counting. When the morning of the ninth comes, begin that day fasting."I asked, "Is this how Muhammad ﷺ used to fast it?"He answered, "Yes."
حدثنا معاذ بن معاذ حدثنا حاجب بن عمر حدثني عمي
أتيت ابن عباس وهو متكئ عند زمزم فجلست إليه وكان نعم الجليس فقلت أخبرني عن يوم عاشوراء قال عن أي باله تسأل قلت عن صومه قال إذا رأيت هلال المحرم فاعدد فإذا أصبحت من تاسعة فأصبح منها صائما قلت أكذاك كان يصومه محمد ﷺ قال نعم
Its isnad is sahih. Muaz bin Muaz Anbari the hafiz is the judge of Basra, and he is a trustworthy imam, the ultimate authority on rigor in Basra. Hajib bin Umar Thaqafi is trustworthy, Ahmad and Ibn Main and others declared him trustworthy, and Bukhari gave his biography in al-Kabir 2/1/74. He is the brother of Isa bin Umar the grammarian. Hakam bin Aaraj is Hakam bin Abdullah bin Ishaq Aaraj, and he is trustworthy, Ahmad and Abu Zura and others declared him trustworthy, and Bukhari gave his biography in al-Kabir 1/2/330.Muslim narrated the hadith in 1:313 through Waki from Hajib bin Umar, and through Qattan from Muawiya bin Amr from Hakam bin Aaraj. Abu Dawud narrated it in 2:303 through Muawiya and Hajib, both from Hakam. Tirmidhi narrated it in 2:57 through Waki, and said, the hadith is hasan sahih.