Sahih Muslim Book - 13, Hadith - 2664

وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى شَيْبَةَ حَدَّثَنَا وَكِيعُ بْنُ الْجَرَّاحِ عَنْ حَاجِبِ بْنِ عُمَرَ عَنِ الْحَكَمِ بْنِ الأَعْرَجِ قَالَ انْتَهَيْتُ إِلَى ابْنِ عَبَّاسٍ - رضى الله عنهما - وَهُوَ مُتَوَسِّدٌ رِدَاءَهُ فِى زَمْزَمَ فَقُلْتُ لَهُ أَخْبِرْنِى عَنْ صَوْمِ عَاشُورَاءَ. فَقَالَ إِذَا رَأَيْتَ هِلاَلَ الْمُحَرَّمِ فَاعْدُدْ وَأَصْبِحْ يَوْمَ التَّاسِعِ صَائِمًا. قُلْتُ هَكَذَا كَانَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- يَصُومُهُ قَالَ نَعَمْ.

It was narrated that Al-Hakam bin Al-Araj said: “I came to Ibn Abbas [may Allah be pleased with them] while he was reclining on his Rida at Zamzam and said to him: ‘Tell me about the fast of Ashura.’ He said: ‘When you see the crescent of Muharram, then count, and fast on the ninth day.’ I said: ‘Is this how the Messenger of Allah used to fast it?’ He said: ‘Yes.”