Chapter on the Recommended Fasts of the Year
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 4
وَ فِي رِوَايَةِ إِسْحَاقَ بْنِ عَمَّارٍ عَنْ أَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ قَالَ: «إِنَّمَا يُصَامُ فِي يَوْمِ اَلْأَرْبِعَاءِ لِأَنَّهُ لَمْ تُعَذَّبْ أُمَّةٌ فِيمَا مَضَى إِلاَّ يَوْمَ اَلْأَرْبِعَاءِ وَسَطَ اَلشَّهْرِ فَيُسْتَحَبُّ أَنْ يُصَامَ ذَلِكَ اَلْيَوْمُ ».
In the narration of Ishaq ibn Ammar from Abu Abdullah (Imam Jafar ibn Muhammad Al-Sadiq عليه السلام), Imam عليه السلام said: "Fasting on Wednesday is prescribed because no nation in the past was punished except on a Wednesday in the middle of the month. Therefore, it is recommended to fast on that day."
- Ṣaḥīḥ al-Faqīh