Chapter on the Recommended Fasts of the Year
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 4
وَ سَأَلَ عِيصُ بْنُ اَلْقَاسِمِ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَمَّنْ لَمْ يَصُمِ اَلثَّلاَثَةَ مِنْ كُلِّ شَهْرٍ وَ هُوَ يَشْتَدُّ عَلَيْهِ اَلصِّيَامُ هَلْ فِيهِ فِدَاءٌ فَقَالَ «مُدٌّ مِنْ طَعَامٍ فِي كُلِّ يَوْمٍ ».
Iys ibn al-Qasim asked Abu Abdullah (Imam Jafar ibn Muhammad Al-Sadiq عليه السلام) about someone who does not fast the three days of every month and finds fasting difficult. Is there a compensation for this? Imam عليه السلام said: "A mudd (a measure) of food for each day."
- Ṣaḥīḥ al-Faqīh, p. 90