Al-Hakam ibn al-A’raj reported: I went to Ibn Abbas, may Allah be pleased with him, and I said, “Tell me about fasting the day of Ashura.” Ibn Abbas said, “When you see the crescent of the month of al-Muharram, count the days and wake up to fast on the ninth day.” I said, “Is this how the Messenger of Allah, peace and blessings be upon him, fasted it?” Ibn Abbas said, “Yes.”
Source: Ṣaḥīḥ Muslim 1133
Grade: Sahih (authentic) according to Muslim
Al-Nawawi said, “Al-Shafi’i and his companions, Ahmad, Ishaq, and others said it is recommended to fast the ninth and tenth of al-Muharram together, as the Prophet (ṣ) fasted the tenth and intended to fast the ninth.”
Source: Sharḥ al-Nawawī ‘alá Ṣaḥīḥ Muslim 1133
عَنِ الْحَكَمِ بْنِ الأَعْرَجِ قَالَ انْتَهَيْتُ إِلَى ابْنِ عَبَّاسٍ رضى الله عنهما فَقُلْتُ لَهُ أَخْبِرْنِي 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
1133 صحيح مسلم كتاب الصيام باب أي يوم يصام في عاشوراء
قال النووي قَالَ الشَّافِعِيُّ وَأَصْحَابُهُ وَأَحْمَدُ وَإِسْحَاقُ وَآخَرُونَ يُسْتَحَبُّ صَوْمُ التَّاسِعِ وَالْعَاشِرِ جَمِيعًا لِأَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ صَامَ الْعَاشِرَ وَنَوَى صِيَامَ التَّاسِعِ
1133 شرح النووي على مسلم