Mishkat al-Masabih #1965
Fasting
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «يَا أَبَا ذَرٍّ إِذَا صُمْتَ مِنَ الشَّهْرِ ثَلَاثَةَ أَيَّامٍ فَصُمْ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ» . رَوَاهُ التِّرْمِذِيّ وَالنَّسَائِيّ
Abu Dharr reported God’s messenger as saying, “When you fast three days in the month, Abu Dharr, fast on the thirteenth, fourteenth and fifteenth.”
Tirmidhi and Nasa'i transmitted it.
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About Mishkat al-Masabih
This hadith is #1965 from Mishkat al-Masabih, in the Book of Fasting.