Chapter on the Recommended Fasts of the Year
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 4
وَ رَوَى اِبْنُ بُكَيْرٍ عَنْ زُرَارَةَ قَالَ: قُلْتُ لِأَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ بِمَ جَرَتِ اَلسُّنَّةُ مِنَ اَلصَّوْمِ فَقَالَ «ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ، اَلْخَمِيسِ فِي اَلْعَشْرِ اَلْأُوَلِ وَ اَلْأَرْبِعَاءِ فِي اَلْعَشْرِ اَلْأَوْسَطِ وَ اَلْخَمِيسِ فِي اَلْعَشْرِ اَلْآخِرِ » قَالَ قُلْتُ هَذَا جَمِيعُ مَا جَرَتْ بِهِ اَلسُّنَّةُ فِي اَلصَّوْمِ فَقَالَ «نَعَمْ».
It is narrated by Ibn Bukayr from Zurara, who said: "I said to Abu Abdullah (Imam Jafar ibn Muhammad Al-Sadiq عليه السلام): 'What is the prescribed practice for fasting?' Imam عليه السلام said: 'Three days in every month: the Thursday in the first ten days, the Wednesday in the middle ten days, and the Thursday in the last ten days.' I said: 'Is this all that the prescribed practice includes for fasting?' Imam عليه السلام said: 'Yes.'"
- Ṣaḥīḥ al-Faqīh