I was with Ibn Abbas in the house of the watering-station, and he was reclining on a cloak of his.I said, "Abu Abbas, tell me about Ashura."He replied, "What about it?"I said, "About fasting it."He said, "When you sight the new moon of Muharram, count nine days, then on the morning of the ninth, begin your fast."I asked, "Is this how Muhammad ﷺ used to fast it?"He answered, "Yes."
حدثنا علي بن عاصم أخبرنا معاوية بن عمرو بن غلاب
كنت عند ابن عباس في بيت السقاية وهو متوسد بردا له قال فقلت يا أبا عباس أخبرني عن عاشوراء قال عن أي باله قال قلت عن صيامه قال إذا أنت أهللت المحرم فاعدد تسعا ثم أصبح يوم التاسع صائما قلت كذا كان يصومه محمد ﷺ قال نعم
Its isnad is sahih. Muawiya bin Amr bin Khalid bin Ghallab is trustworthy, Nasai deemed him trustworthy, Ibn Hibban mentioned him among the trustworthy narrators, Bukhari gave him a biographical entry in al-Kabir 1/4/334, and Muslim recorded this hadith from him. Ghallab is with a fatha on the ghayn and a light lam, and it is said to be the name of a woman, his mother Umm Khalid, while his father was Harith bin Aws bin Nabighah of the Banu Nasr. This hadith is a repetition of 2135.