حَدَّثَنَا هَنَّادٌ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا وَكِيعٌ، عَنْ حَاجِبِ بْنِ عُمَرَ، عَنِ الْحَكَمِ بْنِ الأَعْرَجِ، قَالَ انْتَهَيْتُ إِلَى ابْنِ عَبَّاسٍ وَهُوَ مُتَوَسِّدٌ رِدَاءَهُ فِي زَمْزَمَ فَقُلْتُ أَخْبِرْنِي عَنْ يَوْمِ عَاشُورَاءَ أَىُّ يَوْمٍ هُوَ أَصُومُهُ فَقَالَ إِذَا رَأَيْتَ هِلاَلَ الْمُحَرَّمِ فَاعْدُدْ ثُمَّ أَصْبِحْ مِنَ التَّاسِعِ صَائِمًا . قَالَ فَقُلْتُ أَهَكَذَا كَانَ يَصُومُهُ مُحَمَّدٌ صلى الله عليه وسلم قَالَ نَعَمْ .
Al-Hakam bin Al-Arfaj said: "I met up with Ibn Abbas while he was reclining on his Rida at the Zamzam (well). So I said: 'Inform me about the Day of Ashura. On which day is it fasted?' He said: 'When you see the crescent of Al-Muharram, then count, then fast on the morning of the ninth day.'" He said: 'I said: Is this how Muhammad fasted it?' He said: 'Yes.'" Translation Not Available
Book Ref: Jamiya Tirmidhi Book 8 Hadith 754
Web Ref: Jamiya Tirmidhi Vol 2 Book 3 Hadith 754