Chapter on Fasting on the Day of Doubt
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 16
وَ سَأَلَ بَشِيرٌ اَلنَّبَّالُ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنْ صَوْمِ يَوْمِ اَلشَّكِّ فَقَالَ «صُمْهُ فَإِنْ كَانَ مِنْ شَعْبَانَ كَانَ تَطَوُّعاً وَ إِنْ كَانَ مِنْ شَهْرِ رَمَضَانَ فَيَوْمٌ وُفِّقْتَ لَهُ».
Bashir al-Nabbal asked Abu Abdullah عليه السلام about fasting on the day of doubt. Imam عليه السلام said: "Fast it. If it turns out to be from Sha'ban, it is a voluntary fast, and if it turns out to be from Month of Ramadan, then it is a day you were fortunate to have fasted."
- Ṣaḥīḥ al-Faqīh, p. 96