Al-Kāfi - Volume 4 > The Excellence of Fast and Fasting
Hadith #5

5ـ مُحَمَّدُ بْنُ إِسْمَاعِيلَ عَنِ الْفَضْلِ بْنِ شَاذَانَ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ مُعَاوِيَةَ بْنِ عُثْمَانَ عَنْ إِسْمَاعِيلَ بْنِ يَسَارٍ قَالَ قَالَ أَبُو عَبْدِ الله (عَلَيْهِ السَّلاَم) قَالَ أَبِي إِنَّ الرَّجُلَ لَيَصُومُ يَوْماً تَطَوُّعاً يُرِيدُ مَا عِنْدَ الله عَزَّ وَجَلَّ فَيُدْخِلُهُ الله بِهِ الْجَنَّةَ.

5. Muhammad ibn Isma‘il has narrated from al-Fadl ibn Shadhan from ibn abu ‘Umayr from Mu‘awiyah ibn ‘Uthman from Isma‘il ibn Yasar who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘My father has said, “When a person completes a day of optional fasting for the sake of Allah, the Most Majestic, the Most Glorious, Allah admits him in Paradise.’””