Mishkat al-Masabih #1969
Fasting
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَن مُسلم الْقرشِي قَالَ: سَأَلت أَوْ سُئِلَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَن صِيَام الدَّهْر فَقَالَ: «إِنَّ لِأَهْلِكَ عَلَيْكَ حَقًّا صُمْ رَمَضَانَ وَالَّذِي يَلِيهِ وَكُلَّ أَرْبِعَاءَ وَخَمِيسٍ فَإِذًا أَنْتَ قَدْ صُمْتَ الدَّهْرَ كُلَّهُ» . رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ
Muslim al-Qurashi said that he or someone else asked God's messenger about perpetual fasting, and he replied, “You have a duty to your family. Fast during Ramadan and the following month, and every Wednesday and Thursday. You will then have observed a perpetual fast."
Abu Dawud and Tirmidhi transmitted it.
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About Mishkat al-Masabih
This hadith is #1969 from Mishkat al-Masabih, in the Book of Fasting.