Man Lā Yaḥḍuruh al-Faqīh
Hadith 3551924 - 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».
Hadith.1924 - Bashir al-Nabbal asked Abu Abdullah (as) about fasting on the day of doubt.
Imam (as) 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."