Chapter on Fasting on the Day of Doubt
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 16
سُئِلَ اَلصَّادِقُ عَلَيْهِ اَلسَّلاَمُ: عَنْ رَجُلٍ أَسْلَمَ فِي اَلنِّصْفِ مِنْ شَهْرِ رَمَضَانَ مَا عَلَيْهِ مِنْ صِيَامِهِ فَقَالَ «لَيْسَ عَلَيْهِ أَنْ يَصُومَ إِلاَّ مَا أَسْلَمَ فِيهِ وَ لَيْسَ عَلَيْهِ أَنْ يَقْضِيَ مَا قَدْ مَضَى مِنْهُ ».
Imam Jafar ibn Muhammad Al-Sadiq عليه السلام was asked about a man who converted to Islam in the middle of the Month of Ramadan: What is required of him in terms of fasting? Imam عليه السلام said: "He is only required to fast the days remaining from when he became Muslim, and he is not obligated to make up the days that have already passed."
- Ṣaḥīḥ al-Faqīh