Al-Kāfi - Volume 4 > The Case of One Who Becomes a Muslim during the Month of Ramadan
Hadith #3

3ـ أَبُو عَلِيٍّ الأَشْعَرِيُّ عَنْ مُحَمَّدِ بْنِ عَبْدِ الْجَبَّارِ عَنْ صَفْوَانَ بْنِ يَحْيَى عَنْ عِيصِ بْنِ الْقَاسِمِ قَالَ سَأَلْتُ أَبَا عَبْدِ الله (عَلَيْهِ السَّلاَم) عَنْ قَوْمٍ أَسْلَمُوا فِي شَهْرِ رَمَضَانَ وَقَدْ مَضَى مِنْهُ أَيَّامٌ هَلْ عَلَيْهِمْ أَنْ يَصُومُوا مَا مَضَى مِنْهُ أَوْ يَوْمَهُمُ الَّذِي أَسْلَمُوا فِيهِ فَقَالَ لَيْسَ عَلَيْهِمْ قَضَاءٌ وَلا يَوْمُهُمُ الَّذِي أَسْلَمُوا فِيهِ إِلاَّ أَنْ يَكُونُوا أَسْلَمُوا قَبْلَ طُلُوعِ الْفَجْرِ.

3. Abu Ali al-Ash’ariy has narrated from Muhammad ‘Abd al-Jabbar from Safwan ibn Yahya from ‘Is ibn al-Qasim who has said the following: “This is concerning my question before abu ‘Abd Allah, ‘Alayhi al-Salam, about the case of a people who accept Islam in the month of Ramadan from which several days have passed; if there is any fast upon them of the past days or that it is just the day in which they accepted Islam. He (the Imam) said, ‘They are not obligated for anything, not even the day in which they accept Islam except if they accept Islam before dawn.’”