(7) Fasting
كتاب الصوم
(7a) Chapter: Voluntary Fasting - Section 1
(6) باب القضاء - الفصل الأول
Mishkat al-Masabih 2041 
He said that when God’s messenger fasted on the day of ‘Ashura’ and commanded that it should be observed as a fast, he was told it was a day held in honour by Jews and Christians, and said, “If I am spared till next year I shall fast on the ninth.” Muslim transmitted it.
وَعَن ابْنِ عَبَّاسٍ قَالَ: حِينَ صَامَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَوْمَ عَاشُورَاءَ وَأَمَرَ بِصِيَامِهِ قَالُوا: يَا رَسُولَ اللَّهِ إِنَّهُ يَوْمٌ يُعَظِّمُهُ الْيَهُودُ وَالنَّصَارَى. فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَئِنْ بَقِيتُ إِلَى قَابِلٍ لأصومن التَّاسِع» . رَوَاهُ مُسلم
 صَحِيح  (الألباني)حكم :
Reference : Mishkat al-Masabih 2041
In-book reference : Book 7, Hadith 84